I feel like the answer is staring me right in the face, but I have yet to discover it.
I am looking to be able to trigger a LFO or envelope to start, run its course once, and then stop. Specifically -- I have patched up a polyphonic granular device like a Freeze and want to hit a switch to have the frozen sound fade in (and hold if possible) and out at a predefined rate. All ideas are appreciated!
So you want an LFO or envelope to trigger once then stop?
There may be a one shot LFO module - there are so many modules that I may have just not come across one yet.
If not you can patch it yourself.
If using an lfo you'll want to find one with a reset trigger input. Once the lfo goes up and then comes back down wire up a gate to close off the lfo output. You can use any object that tells you when the lfo value goes less than zero like a [> 0] for example.
If using an envelope you'll want one that needs one trigger to run to it's end. Many envelopes need a gate (which is held compared to a trigger than is very short with a definitive end) The simplest form of this is an AR envelope or a decay envelope. You want to send it one trigger to start and then close a gate to ignore all other triggers.
When and how the lfo or envelope receives triggers again is up to you.
I'd show you more with a patch but I'm on my phone. I feel my explanation is a bit half baked.
Also what you're asking about is a modular patching question in the general sense and nothing really specific to the Axoloti so you might find answers in other modular forums, too.
Bumpedidump 
I would really love a oneshot phasor too.
@spacelordmother, did you find a solution for this?
an ahd module? fade in, stay there as long as the trigger is high, go back to zero when released.
