Carl,
They can mount a weapon on the ramp, and if memory serves me right they have door guns in the Air Force (Special Operations) version which is the CV-22, not sure what the Marines mount on their version of the V-22 which is the MV-22. They were just coming into Afghanistan during my tour there, so while I saw them, I never flew in one. No doubt lots of guys in SWJ have first hand experience with them and can answer your question. I do recall during the V-22 troubled development phase that one of the issues was how they should be armed.
Technology is always improving, so this comment may be outdated, but it has been my experience that suppressive fire from door gunners (both window and ramp) is just that, it is suppressive fire that isn't very accurate. I know you don't want to hear this, but if someone is shooting at you from within a crowd of unarmed civilians your options are somewhat limited, unless the rules of engagement allow that type of response.
I don't know if SOF or Marines mounted the non-combatant evacuation (NEO) in Sudan, so I don't what version of V-22 went in, and since the weapons are optional, I don't know if they had weapons mounted. Despite the troubled development period, by most accounts the V-22 does give us some pretty good capabilities we didn't have previously.
video link showing tail mounted weapon
http://www.youtube.com/watch?v=OZN1EBN5z1I
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