You're somewhere that a whole lot of people have been before.
For one thing, (next time) always use an anti-seize product on the pin threads. Permatex for example.
Always use a good tool. Allen wrenches commonly are abused and the resulting damage makes them a loose fit to start with..and that's not a good thing.
Even so, you are likely going to have problems.
I always use anti-seize, I change pads often so it's not a matter of them sitting around for a couple of years (2-3 sets of rear pads a year, easy), and my wrenches are brand new and of decent qualtiy (MotionPro).
Still...last time I went to change pads the head of the brake pin just plain gave out.
Heat? Not a chance, imo, due to aformentioned seals inside.
I ended up drilling thru the failed allen head, using a bit that was large enough to effectively remove the actual straight part of the pin from the threaded portion. That gave me a large enough diameter and depth to use a decent sized EZ-out (Snap-on). It came out pretty easy!
A couple of problems with this procedure: Make sure you don't mess up the caliper threads with too big a bit or an unsteady hand. Use the smallest bit you have to to separate the straight pin from the threaded portion. The more meat you leave for the EZ-out to bite into, the less chance of damaging your caliper.
Then get some aftermarket pins (Moose makes them. I don't know if they have the correct size. Steahly has them in his catalog). They will have a hex-head you can use a socket (6-point of course) on as opposed to an allen head (socket hex).
I tried the torx approach. Not nearly enough metal left to bite. Tried drilling into the pin to get a better torx bite. No joy.
Good luck!