I am actually pretty close to a working concept, just "hemming and hawing" over the deep implementation specifics, as I'm trying to implement Portable Object support entirely as a plugin for my ActivityPub server project. I should be able to achieve demonstrable results by next weekend.
Invariably, it should be very possible to have a portable profile, while still being understood by existing implementations, it's just going to end up with URLs getting a bit longer. Within the proposed extension, there is simple remark for compatibility with existing implementations: https://codeberg.org/fediverse/fep/src/branch/main/fep/ef61/fep-ef61.md#compatibility
Essentially what this could look like, is where instead of purely DID-based identifiers, that the DID is appended to the "DID resolver endpoint" of their main server. Portable Object-aware servers could then strip the URL down to the plain DID, and store it in DID representation (with the original URL as an alias to it). Meanwhile, conventional servers could still be able use the resolver-based URL just like any other typical ActivityPub server.
For practical example, of a DID into a resolver-based URL, the transformation would be from a portable identifier of:
did:ap:key:z6MkrJVnaZkeFzdQyMZu1cgjg7k1pZZ6pvBQ7XJPt4swbTQ2/posts/2024-03-30/123
into https://example.social/did/did:ap:key:z6MkrJVnaZkeFzdQyMZu1cgjg7k1pZZ6pvBQ7XJPt4swbTQ2/posts/2024-03-30/123
It's ugly, but it works, but it can also pressure vendors to add support, and thus be able to make use of more compact representations in the database too, as another benefit.
Meanwhile, in regards of Bluesky, my big concern is that users don't even control their identity. Whereas in my current understanding: users don't have access to the private keys associated with updating their did:plc
identifier, which controls what PDS a user is even on. And even if Bluesky gave you control to change your PDS in the future, they would still ultimately hold control over your identity, if they hold exclusive control of those private keys.