[mariusor]

I am not convinced that it can't be done. (Saying this also as someone that wrote a spec compliant ActivityPub server: https://github.com/mariusor/fedbox)

Nothing says you can't extend an actor to provide for a did: based identity which gets stored wherever you want to. I think the main dev of Mitra[1] is someone that's exploring very strongly in this direction.

[1] https://mitra.social/