building an ascii to midi AU plug

so ive been playing a bit with mainstage, and more specifically i
think im becoming hooked to some of the logic plugs(the epiano is
especially nice)

rax has been bought(again) and one if the new features is supposed to
be ascii control of midi functions

but im starting to think that i may just stay with maintage for live usage

and as far as i know, mainstage doesnt do anything like that, and its
unlikely that itll be an added feature, since its more 'pro' than that

what i would like to do is to create an (AU) plugin (to work in
mainstage/rax) that will convert key messages into midi note messages,
with some sort of dropdown menu interface to make it more openly
usable, to control with a rehoused 10key usb numpad 'foot controller'

making that in max shouldnt be too difficult

however, i have no idea how to go about making something like that
into a plugin, or if it would even work (does 'key' work outside of
max, in a plugin, or if the plugin/app is minimized), or if its
possible to do so with just max (do i need pluggo for this?)

i did a forum search, and a quick google search, but only really found
the examples in the pluggo product page, which are audio processing
plugs

so i guess theres 2 questions:

is it possible to make something like that at all?(using max)

how does one go about building the plug specific architecture?