Don't forget aluminum foil.

So here's my thought:

Once you've soaked the threads for the night and have found an allen wrench that fits tight in the rounded hole take a bit of aluminum foil and put it inside the rounded hole of the crank cap and then force the allen wrench into the rounded hole. Slow steady pressure until you've either bottomed in the crank cap hole or as deep as you can go, if it's a plastic cap and not an aluminum cap use extra care as you seat the allen wrench into the caps rounded hole not to crack it with too much pressure. The goal is to have the aluminum shim up tight on the edges of the allen wrench against the inside of the rounded out crank cap hole. Then bring up your pressure slowly on the lever arm of the allen wrench and see if you can get the threads to give and move before you overcome the friction of the allen wrench edges biting into the shim of the aluminum foil against the rounded out crank cap.

Good luck.