Come to think of it, is the payment protocol really the place to put this instant provider signature

Yes it's the right place. The original attempt at this concept was in fact called green addresses and the idea was you could identify a spend from a trusted wallet by checking which keys were being used to sign. But the problem is, lack of privacy. Everyone can see what wallet provider you use.

Also it'd be inefficient to have in the chain. There's no reason for the extra signatures to be there: double spend risk is something only the recipient cares about.