This is an amazing addition, and it could serve a lot of other projects too.
FEP-0ea0 already specifies exactly this: a payment Link (rel "payment") in the actor attachment array. Emitting both that and a PropertyValue keeps it visible on Mastodon as well.
FEP: https://codeberg.org/fediverse/fep/src/branch/main/fep/0ea0/fep-0ea0.md