The V-22 is not without its strong points as a support aircraft. However, CAS is a specialized role for which it will never truly be ideal. That's not to say it wouldn't be feasible to deliver weapons with one, once modified appropriately.
The good news is that you could likely bring it up to roughly attack-helicopter levels of armament. I estimate you'd get roughly AH-1 levels of firepower, but with a transport capability and over significantly longer ranges. Essentially, you'd end up with a sort of modern, squishier version of the Mi-24.
The squishiness is a significant concern though. Compared to virtually any rotary wing platform, the V-22 is large and loud. It's easier to acquire by sensor, by sight and by ear. It's also not particularly resistant to ground fire. In that sense, it bears some of the some limitations of UH/CH platforms or "C" class fixed wing A/C.
In that sense, a weaponized Osprey is actually competing to be a newer-bigger MH-60L Direct Action Penetrator. You could also view it as an update to AC-47D (Puff the Magic Dragon).
There is certainly a niche there, but as a general purpose, high performance, ground attack A/C it falls short. As noted above, external stores would degrade performance and there would be limitations imposed by the airframe concerning capacity. When you start taking weapons off the table, you start limiting capability. The speed, altitude and maneuverability of the platform will never be ideal for integrating into fixed-wing A/C formations, nor will they confer the ability to out run or out fight even the most outdated of adversary fixed-wing A/C in an unescorted role (an A-10, for example, is more than an match for a V-22 air to air).
My verdict: - Viable long range, rotary-wing, general purpose support/strike platform - Not a true/ideal replacement for fixed-wing CAS capability
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.
CAS, no. Mini-gunship, firing standoff weapons in coordination with marines on the ground, quite likely will happen. The V22 should be able to substitute, albeit with less effectiveness, any role the C130 performs for the marines(cargo, tanker, gunship, ISR). There will be scenarios where C130s can't get to the airspace but V22s are embedded with the MAGTF and will always be there.
The V22 will be contributing as a CAS platform for the USMC along with KC130s until the service finds a suitable platform to replace the A10. Marines are actually running out of fighter jets. At this point, only about 87 of its 276 F18's are serviceable at any time.
For any flying arm, roughly 30% availability is really bad! And the A10 cost \$20,000/hr flight time and is increasing constantly because of the aging of the airframe. The services gambled with the F35 and lost, so until the F35 is combat worthy or is scrapped and a suitable replacement is found for CAS role, the under-funded marine corps is making due with what they got.
The Harvest Hawk modification kit for all the services KC130s, adds an electronic jammer, a Target Acquisition and Designation System, and hard-points. Load-outs have featured as many as four AGM-114 Hellfire and ten AGM-176 Griffin missiles. They’re hardly as sophisticated and capable as AC-130U/W/J gunships, but the modular kits cost far less(\$10 million vs. \$200 million).
The Osprey’s weapons kit is less defined. With hard-points for targeting equipment and laser-guided missiles—APKWS or Viper Strike. And they will be capable of such functions, though just as DanS said, "albeit with less effectiveness".