Hi flowli,
I had the same idea a while back. I think z-push is absolutely a perfect option to realise (mobile) sync options in FengOffice.
I didn't really started hacking yet, but gathered a whole lot of conf's from similar (open source) collaboration suites already using z-push (to know how to get started.. because documentation from z-push is not that great in my opinion). If you're okay with it I’d like to join in and make this thing happen, it's the one and only thing I really miss from FengOffice and all former attempts for syncing seemed to have died off (It's not even in the 'Sponsor a Feature' list any more).
Cheers,
Joost