If there's still enough of the head left to get some torque with a fresh allen key, try heating the wheel up. I did this by propping it on bricks 10" above a low flame on the gas hob for 20 minutes or so. I insulated the seal for the wheel bearing with by taping over with a few layers of newspaper to prevent damage to the rubber. You just need to get it hot enough to be uncomfortable to touch say 50 to 60ºC, any loctite type material will soften and the alloy will expand more than the steel of the bolt making it a looser fit.
Other options are: weld a length of steel to the remains of each head, though this could risk damage to the alloy as localised heating might be too extreme... or simply drill the heads off, lift the old disc off and hope that once the tension is off the bolt the remains will respond to a good soaking in WD40 (or whatever your preferred penetrating oil may be) and the bodger's favourite tool: the mole grip.
