My 10w halogen trailrat head is in dire need for a new battery pack, but I figured that it might be fun to mod the head with a high-output led such as a p7 or something. My question is, has anyone done such a thing?
Yes, that's one way to do it. All the ready-made MR11 LED lamps that I've seen take a 12V supply, which I think differs from the Trailrat's battery pack. But you don't necessarily need to use the MR11 receptacle.
Regardless of how you did it, I suspect you'll have a hard time getting enough heat out of the plastic housing to maintain a reasonable junction temperature on the emitter. If you find a clever way, please post it. I've got an old Niterider myself that I would like to convert to LED, if I could find a convenient thermal solution.