I recently suggested something similar in another related, more focused, pitch thread.
I think it would be better to annotate the conforming member, rather than the type (and always use conformer), since it would allow a type to conform to many protocols, but forward each protocol conformance to different children. See a motivating example further down in the mentioned thread, where Task conforms to both Collection and Comparable, by delegating to different members.
Anyways: Bikeshedding syntax aside, I think this is a problem worth discussing.