I've hammered a spline socket onto a stripped internal-hex (allen) bolt that was holding the crank pulley on an older VW Jetta TDI. I couldn't use a vice-grip or other pliers on it because the bolt was recessed. Since the outer surface of the bolt was completely round from the start, the only way I could do it was to hammer a socket onto it. It DID work, though. I suspect it would have worked just as well with a 12 point socket.
I replaced all those allen bolts with standard external-hex bolts after removal.