you can get stud extraction tools where by you drill a hole thru the middle then wind the bit in and it bites in then u use that to remove it but be aware if the bolts are seized in then potentially i have had them snap on me before and their a bitch to drill out after!! probably easier as you have said tbh cos then u can just get a good set of mole grips on it after and it should come out. By drilling the head off the bolt you've removed the tension on it so should make it easier!

The other idea would be to hammer the next size up spline socket but the headbolts are torqued in very tight so probably wouldnt work, have done it that way on normal spline/torq bolts before