Overlay Notifications

On the other hand: I'm working on a GUI that doesn't have a separate list: you just start with the top message, and scroll down to the next message. And I'm not sure how that would work with custom displays...

That sounds neat! I guess in that case your new messagesgui app (that is the list) just always says 'yes I can display this message'. I guess it still leaves open the possibility that some other app (like navigation?) might be installed and would want to handle the message instead, in which case I guess that is still fine?

Another thing we might look at is response actions

Yes, that'd be a neat one for later.

get GadgetBridge to pass action buttons

I hope so... I seem to recall it can pass some info already?