[sc_rufctr]

You could drill out the head with a drill the same diameter or larger than the shaft of the bolt.
AT some point the Allen head would separate from the shank and take it from there.

You'd be able to grab the remain shaft with Moll grips or something similar.

OR... "Slot" the head with a Dremel tool then use a large flat blade screw driver to undo it. I don't think this will work but it's worth a try ... try this before drilling it out.