From what I can tell, the Allen head in the clamp looks like the head of a standard 1/4-20 socket head cap screw that has been press fitted into the recess hole in the aluminum clamp. The knurling in the head helps to hold it into place. This is very similar to the way that the lug nut studs are attached to the brake drums and rotors on cars.
Unfortunately, once this Allen head strips out and breaks free, it would seem that the only recourse is to order a replacement. However, even with a stripped Allen head however, the clamp itself should remain functional. You just won't be able to use an Allen wrench to tighten or loosen it, but pliers, the green jar lid gripper, or the strap wrench all mentioned above will work.
I never use an Allen wrench to tighten these clamps, even with my heavy 180 EDT imaging and guiding setup. If I need to torque them a little more, the jar lid gripper works really well, and it also works really well for loosening the clamps. Before using the jar gripper, I used to need an Allen wrench from time to time to loosen the clamps.