At this point I'm fairly certain that FEP-ef61 can co-exist with Mastodon. My preferred solution is to use a resolver endpoint URL as object ID instead of a plain DID URL:
https://social.example/.well-known/apresolver/did:ap:key:z6MkrJVnaZkeFzdQyMZu1cgjg7k1pZZ6pvBQ7XJPt4swbTQ2/path/to/object