On some stripped allens, I've had good luck tapping in a slightly oversized Torx bit. Not sure what sixe you'd need for the steering wheel bolt, probably a T50 or larger.
I'd try tapping on the bolt itself a bit with an air hammer and a small tapered bit, or with a smaller flat-faced bit.
If you weren't using air tools before, you'll need to to do this.

Gilly