They typically do this in India - definitely 6E does it and in pretty sure AI too - when they have a remote stand (which is pretty much most, if not all (at least all that I've been on) domestic flights from BOM). 6E even have signage on the busses and ramps to the planes that rows 1-15 board from the front, 16+ uses the back door.

It's not necessarily more efficient this way, however. Some of the bus rides to the remote stands take so long, they probably could board from the front door only in half the time.

Seriously though, IIRC, UA did a test with this at a gate in DEN several years ago - and it lasted right up until the first time an aircraft was damaged by the rear bridge.