I would coat the stripped thread with JB weld (industrial epoxy) then retap, or if retapping is not an easy option screw in a M5 bolt covered in a suitable release agent (silicone oil) while the epoxy is still curing. Some of you may remember that I used JB weld to attach a LH downtube shifter boss to a steel frame, that was over a year ago now and it's still very firmly attached
Hmm, this sounds like a method that would involve less powertool use on my part, which is a good thing...
When you say you coated the threads, how did you do that without completely filling in the hole? And what sort of silicone oil did you use? (Looks like there are many types.) I guess if it all went a bit wrong I could still drill it out (thanks for the other suggestions, feanor, QG, et al). I'd prefer to avoid that option if possible, given that it's brazed in rather than a rivet.
I'm based in York, if anyone nearby has the tools and a grumpy beard (I gather this is a requirement for this sort of job).