Anybody done this?
Why? to use a lightweight rear diff without housings etc, with a front engine format for drifting events. vtwin layout merits transverse mounting due to packaging rqmts.
Anybody foresee problems, as long as chain is kept in tension

You're talking about a pretty long chain run there... You'd need some kind of guides or idlers I would have thought.
DP Cars has done something with long chains for the drive system I believe. Google it.

Cheers

A short chain drive to a 90° gearbox and then a prop to the rear would probably be your best bet, but the gearboxes are big money!
Other options include shaft drive v4 or a shallow twin like the ktm or aprilia

Or just make the car left hand drive and lose the passenger footwell!