I mentioned HubZilla over at the Nexcloud boards, and it seems like it would be possible to allow federated file-sharing?
https://help.nextcloud.com/t/hubzilla-an-interesting-selfhosted-social-network-companion-to-your-nextcloud/Nextcloud uses ActivtyPub and SabreDAV for its federated file sharing features, and for example Pydio (another private cloud system) has implemented the protocol.
It looks like this could be a low hanging fruit for Hubzilla as well?