The other way is to heat the old derrailleur bolt up. But then you're risking the flame from the propane torch scorching the paint on the frame.
I'd go with operator's penetrating oil suggestion. Liberally applied to the area and allow to sit for a day. Another thing to do is use a pin punch and hammer to rap the derrailleur bolt a few times. Preferably one that fits down into the hex of the head. Then hit it like you would a good size nail a couple of times. Between the raps and the penetrating oil it should snap loose with the good fitting allen key without too much more effort.