I was wondering whether the Piefed Zulip could be bridged with XMPP, through, for example, Matterbridge.

I'd like to join it, but I really don't need another client running if I already use a free and federated protocol. :D

Also, XMPP aligns pretty well with Fediverse ideals. :)

Zulip hasn't really taken off among the user base so we're keeping it around for now for internal comms but I don't see myself putting extra energy into it at the moment.

Zulip is just a tab in your browser. The threads management really helps to keep conversation going on over time. I've used Matrix, XMPP, I prefer better communication features over federation.

People who want to comment on Piefed issues have to create a Codeberg account, that's not federated either.

The Zulip mobile app also has multi account, as opposed to Element X who still doesn't support it

Don't see zulip as a chat, it's a forum with a chat interface. So yes, making another account for a specific forum is not that bad.