libpd seems to work real well on android.
https://github.com/libpd/pd-for-android.
i spent <30min messing with it but could load an arbitrary patch and control it from android activity w/ no problems.
so the question is how easy to make virtual COM port appear on android. i haven’t done any host mode stuff, but here is low level host mode API:
http://developer.android.com/guide/topics/connectivity/usb/host.html
maybe (probably) there is already a library somewhere to give you an easy serial interface on top of that.
UPDATE: here’s a sample Activity for serial USB using the low-level host API, and standard perimissions management from android OS. it looks simple enough… but i haven’t tried running it, and the code is old (2011)
[ http://android.serverbox.ch/?p=370 ]
then on top of that it’s the usual monome protocol parsing as in libmonome/aleph/etc
UPDATE 2: i think this is the sample project i used to build with libpd under android studio:
[ http://www.journal.deviantdev.com/sample-libpd-android-studio/ ]