currently the only way 'users' can do this is by including header files that are located in the same directory as the object.
(you will see I do this with my push object... I include both C and H files into the object )
the filters you mention are different, this code is compiled into the firmware, so the unless you plan to build your own kernel version not really viable. (and then you wouldnt be able to share objects/patches with others)
this is far from ideal, so Ive create a new feature for the next release, which Im testing now, which Ive tentatively called 'modules' , this basically gives developers the ability to use/package C source and headers, as a compilable units, in a way I hope will allow both reuse, and also some level of versioning... its early days for this though, still experimental and will be a while until release.... I mention just so you know its being 'considered'