I'm missing the point here. Using a Teensy or some other device just ends up with a Midi splitter. For that purpose why not just get a MidiThru from Midisizer? Send Midi-Out to it from the Axo UART. You can power it from the Axo and also put it in the same enclosure so it looks like an all in one unit.
On the bandwith question. I have a Midi-Thru, which has 1 Midi-In and 6 Midi-Out and I drive all those Out's from a MidiboxSEQ simulatenously and it never drops a note or loses clock.
yeah, understandably the focus has been on how...
but as i mentioned in the first post:
the primary purpose is intelligent routing of midi, using the axoloti to store CC mappings, program changes and routing to various devices.
the midithru only covers output, where as ideally, Id like both input and output, so that the axoloti acts as a 'intelligent hub'.
I think some interesting options have come up here. I need to consider each, and determine the amount of software development on each... then I can order some parts 
thanks all for your suggestions.
axoloti has 3 ways of getting midi in as fara as I understand, regular midi, usb midi through host connection and attached via usb to a hostcomputer, but the latter connection would work connected to an arduino with a usb host shield, wouldn't it?
to get 7 midi connections, use 2 arduino mega's, one with a usb host shield, and connect the axo's usb to that and get 3 midi that way, use the other mega with flashed usb-chip to make it into a usb-midi-device and get 3 more midi ports that way, and lastly the regular midi port is another, so that makes 7. if you manage to use another hw-serial on the axo for midi, use another mega and that's 3 more ports and 10 total. it should work, shouldn't it?
there is also a small clone of the mega called the meduino mega2560 (check eBay) if you are worried about the size of the thing.
also, the usb host shield supports usb hubs, so that is a way to get loads of usb-midi connections (I'm away for holiday, so I can't check this myself right now, but it's supposed to work), there is a mini version of the host shield that works with teensy and the regular for arduino
It seems like you're overly complicating things? Outline exactly what you want 
if you want 3 extra midi in/out ports on the axo, the simplest and cheapest would probably be a teensy 3.2, it's got 3 hardware serial ports and the arduino library will allow you to use a pair of pins for software serial any of which could be used to connect to the axoloti, leaving 3 midi in/outs + the onboard ports. If you want 7 ports then 2 mega boards + onboard would do that and you wouldn't really need any host shields or to involve usb at all.
my point is then you take advantage of the factory axo midi handling that is already there in the firmware, you don't need to write any additional midi code (on the axo, you'll have to do it on the arduinos naturally) using an extra uart for midi takes additional processing
I don't know if there is anything actual to gain here, but it's just a though, and, it could possibly give you 10 midi ports
but it's probabely total overkill 
as always, many ways to get to a similar place...
from reading the alternatives (and there are I'm sure many more), it will come down to how much extra hardware you want to buy, how much soldering you want to do, and how much coding...
so solution will be different for different people.
I'll have to think a bit, as I'm currently coding for not only axoloti, but also now bela (beaglebone), so do I want to add arduino too
... and I think i was kind of hoping for more of a hardware diy project, rather than another software one, but hey i guess everything comes down to software these days.
I'm torn between using the gear I've got and making something, I've got an arturia minilab (25 keys, 16 rotary encoders, 8 pads, across 8 or 16 banks), emu xboard (49 key, 16 pots). Ultimately I think I'd like at least a bunch of pots/encoders and buttons on a panel hardwired to the axoloti.
Ah. To me the obvious platform to get multi-IO and intelligence is the Midibox platform. They have very flexible IO options and MIOS32 already contains high level functions for doing all of this stuff.
I looked at midi-box in the past, it looks really well done but for my smaller needs so far the arduino platform has handled it all really well, it's great to have options!!
What extras can the midi-box do mongrol? I'm intrigued what @thetechnobear wants to make as well?
I've got a few more bits of midi gear here that I haven't mentioned that I think would make some really nice control surfaces, like a behringer cmd pl-1, lots of buttons, rotary encoders, platter, banks.
I haven't had a chance to play with the axoloti properly yet but ultimately I'd like to see a comprehensive set of functions to deal with all of the different midi data-types for rotary encoders, hi-res pitch/nrpns pots/faders etc.
basically, something that act as a router for midi, with a fair amount of configurability in routing and mapping midi messages.... that can then be saved and recalled. (either as patches, presets /data files) .
don't get me wrong, i know exactly how to do this... and I also have multiple axolotis, which I can chain... I could also potentially you a BBB/Bela as a 'hub' - also of course, I could just go buy a iConnectMidi4+
so really this started, see first post, as more a simplification of this...
could i do it with one axoloti board?, with a simple 'daughter board' that plugged in. a cheap and cheerful solution... with just a bit of coding on the axoloti.
(as by-product , i also thought it might be interesting to other axoloti users.... again a reason for simplicity)
the reason I've hesitated in USB connection, as i mentioned, is I do have USB class compliant midi devices, so i need at least one usb port for that.
I will admit, the lack of USB hub support on axoloti, is a minor 'issue', which actually makes the USB shield quite interesting. (also it seems to support USB 2.0 which is useful)
(btw: @johannes the usb shield 2.0 , is another good source for looking at usb drivers, especially for inspiration for the usb hub support. when my 'desk' clears a bit, I may take a look... though its still a non-trivial task)
Midibox, yup very familiar with this, and really if I had time to build one, id be tempted, as much for the build experience as anything else... but really I don't need much of what it provides, and having checked the manual, the software, does go in a direction I'm interested in. (Im much more interested in the Squarp 
) yes, I know its open source... but I don't have time for coding another platform
(also, I don't think its supports usb hubs)
... that said, the code base is awesome, Ive checked it a few times when doing things on the axoloti usb midi implementation.
upshot is... when I'm placing my next order for electrical components, I'll make a call... try to find a balance on something thats not too coding intensive.
but for now I'm tempted to use Bela, and then use axoloti as daughter boards... its a bit 'heavy' for a simple midi router, but is in line with my 'main project' anyway, but it'll only become an issue when I need to use the other axoloti's for synthesis and fx.
this thread has really opened my eyes up to arduino and teensy options. I knew of there existence etc, but never really looked into what they could do, they do provide some nice small package options. Im sure at some point I will do something with them. so thanks for the ideas and information.
I agree that the arduino platform is easier to developer of. I personally find Teensy 3.2 just awesome. The Midibox platform though already has Midi IIC boards developed and integrated that supports up to 4 boards with 8 ports. All of them can be individually configured to be in or out on the fly.
http://ucapps.de/mbhp_iic_midi.html
Theoretically, since the boards are i2c, you could attach them onto a Teensy (or arduino).
@mongrol t was because of the teensy 3.2 that I ended up getting an axoloti, to follow on from that, as I've got an stm32F4-Disco board sitting here, it looks like this thread and the axoloti is going to finally make me take a look at midibox and build a mhbp_core board, I've got most of the stuff here, I just need a couple of ICs.
I bought the teensy 3.2 to play with the audio lib, I figured if I can't build something myself (probably due to lack of coding skill) I can use the 3.2 in one of the many eurorack modules it's been used in.
@thetechnobear it'll be interesting to see what you come up with
Which if you just want this kind of functionality (vs making the axo do it), is a great piece of kit, especially because it has a virtual audio interface for the USB devices attached.