Managed to get a Pure Data patch running in an Android app. Their lib is nice and seems actively maintained. Need to target an older Android version though, some maintenance to do (actually it's a one-liner, a PendingIntent needs a mutable/immutable flag, I might have a go at contributing)
This is going to be awesome
This'll work, the Pd patch is very basic at the moment so there's some pops and clicks, easily fixed: https://www.youtube.com/shorts/0Zk6rc8JqIE
Resonance for Android

YouTube
Eliminated the pops and clicks. Also implemented midi support, so this should be able to drive any synth. Next to add note editing, visitor size/range, and patch loading and saving.
I reused that volume control I invested a load of time in. Almost works without too much thought, just needs a little tidying up.
Multitouch! And note selection. All that's left now is patch load/save/crud, and some UI polish, plus a setting to toggle midi mode (which I've not actually tried yet)
I have no idea why yet, but F#4 doesn't produce any sound... it's a bizarre bug.
Improved midi note picker, splitting into octaves makes finding notes much easier than just having a grid with 127 cells.