@megalodon knowing how broken IAPs on PlayStore are, I bet it doesn't make sense to try to open it up.
Much more so for legal reasons than any other.
Playing the devils advocate, would mastodon.social have to send the donations manually to other "registered" instances? Who is going to keep track of those finances? Who would eat up the transfer fees?

I don't see how donations API would work with the administrative requirements. Maybe someone can expand on those suggestions?