> How that works, especially with freedom of movement from the rest of the EU, is anyones guess

This was already answered. Germany has a welfare-system which is basically a restrictec UBI, and there were non-german eu-citizen demanding access to it and went as high as the European Court of Justice.

Result: they have no right to claim social welfare outside their own country. Freedom of movement in EU has limits and you must behave like a proper local citizen (like working there for some time) to get access to other countries systems. And even then benefits are time limited.

Presumably it would be citizens rather than residents.

I'm not sure that would be legal, would it?

I think there was a test case in 2015 that said you could have tests that a resident was not there just to claim benefit, but I think the point was that it was illegal to just completely restrict a benefit to your own citizens.

It would probably be modeled the same way as social benefits are modeled. A mixed of citizenship and residency...