I think I understand this a bit better and think you have a bit more of a problem than you're going to enjoy fixing. Correct me if I'm wrong, but it sounds like there is a threaded piece that is normally welded inside your hollow aluminum frame. This piece was broken away from the interior wall of the frame, and now the threads and nut are just rattling around in the hole. Since the frames are stamped sheet aluminum that is then welded together, I don't think a helicoil approach is going to work, simply because the thin aluminum isn't going to be thick enough to seat new threads. My gut reaction to this is that your frame needs to have some sort of new threaded widget welded in place at the point of failure. I would get an opinion from a dealership on this. Bottom line, tho, is that it doesn't sound like it's possible for a home grown fix. Whatever you do, don't cut off the head. Otherwise, the remnants inside the frame will drop in and rattle 'round forever and ever.
{edit] Ack - just saw that this thread was a year old. I wonder what actually ended up happening, though....