As you say, Sandiego89 - about the chin-mounted gun on that publicity picture - it seems to leave no place for any services to be run to the gun, in addition to any issue with balance. The region of space outside the front door that needs to be avoided when shooting is large, and depends on the tilt angle. Under the floor does seem to be the natural place for mounting a gun.
Conventional armed helicopters might be more agile, but they can't accompany an Osprey as far or as fast as it's intended to reach from offshore. Could a dedicated armed Osprey look after its companions more easily and effectively than Harriers in the USMC's plans for using it?
Is it a possible 21st century Hind, since it's already got an improved version of the rear-facing gun for Afghan use?