You can overwrite the screen in Android. You can replace the home screen in Android. You can scrape running apps in Android. There's a high degree of flexibility
Unfortunately this is probably one of those problems where the cost of building the app is relatively high (because the domain is complicated to get right!) and it would be hard to make too much money on this
BTW do you remember why making a browser extension is a bit involved, and publishing, even more so? Trusting someone else's code. Copy -pasting it from a message board may end up in a problem.
It looks dead in that repository though.
But I would totally be for some much simpler packaging scheme for native apps/"scripts". It's kinda insane how much money is going into Android from Google but all the dev tools are basically just Java stuff. I really wonder what Android could be if a more lisp-y approach existed as well