Could the V-22 Osprey be used as a close air support platform?
Can it be? Sure.

Let's take a look at what it would be replacing. 

The A-10 was designed from the ground up to be a platform for moving the GAU-8 into range of the target. 


It has good low speed flight characteristics. 
The engines are mounted high and to the rear to protect them from FOD (both debris kicked up by the GAU-8 and hostile weapons fire). 
The cockpit is within a 540Kg titanium armored tub to protect the pilot and critical flight systems. 
It has wing hard-points for carrying additional munitions should the GAU-8 not be sufficient for the current mission.


Why won't the Osprey be adapted for CAS?


Because it travels slowly, it's susceptible not only to MANPADS and other traditional AA systems, but simple small arms fire becomes a threat.


For example, the Douglas A-1 Skyraider (which the A-10 replaced) - a "propeller-driven design was also relatively slow and vulnerable to ground fire. The U.S. Air Force and Marine Corps lost 266 A-1s in action in Vietnam, largely from small arms fire." (Source, emphasis mine)

Those huge props (38' diameter, which is smaller than the optimal 43' (Source)) are a very vulnerable target - damage to them may make the aircraft unflyable. 
Hits to the engine rotation system that may not cripple the aircraft may cause it to have to land with the props in the vertical plane instead of horizontal. They're designed for this without destroying the rest of the aircraft, but that means thorough inspections and expensive prop replacement prior to its next flight.
It could have an armored cockpit surround retrofitted, but it's not designed for it.
It could have hard-points mounted, but they would need to be integrated with this swing-wing mechanism which would significantly increase the design/build cost and complexity


The swing-wing (used for stowing the plane on a carrier) could be eliminated, but that would defeat one of the main uses of the aircraft.



It's highly likely that the replacement for the A-10 (current thought trends toward the F-35, but that's contentious) will be more similar to the A-10 itself. That kind of role has some unique requirements that can be fulfilled by a more multipurpose aircraft, but not as well.