I wonder if someone could work on a C2S (Client to Server) project to see how it could work https://www.w3.org/TR/activitypub/#client-to-server-interactions
I think the breaking part is largely the inability to define things around uploading data (#ActivityPub doesn't define one but there's mention here on some approaches https://www.w3.org/wiki/SocialCG/ActivityPub/MediaUpload)