Is there enough of the tap sticking out from either side to get some pliers on it? If you can get to it, you may be able to back it out. I don't know how much room you have if if you have the welding skills, but sometimes you can weld an extension on to the broken piece and then still back it out.
Finally, if needed, have a welding shop or body shop cut the piece out and then fill the void or patch the area so you can try drilling and tapping again.
