SOLUTION: Destroy the thing to get it out.
DIFFICULTY: Rather easy, with a variety of methods:
The simplest is to take a Big Damn drill bit to it and drill the head off. This leaves a stub, but you can easily get the rotor off. You can simply leave it this way, using it as a locator for the rotor. That's what I do.
Those who favor brute force will whack the rotor with a Big Damn hammer to break the head off the screw and then either leave it as is or follow the end of the next procedure.
The most anal retentive types will start out by using rust penetrants and an impact tool to possibly break the thing loose and take it out before they go ahead and drill the damn top off. They will then insist on grabbing it with vise grips to get it out and replace it with a new one - properly treated with antiseize compound, of course. Meanwhile, I'm already driving down the road ...
If it worries you not to have that screw in there, consider these two facts:
Some Audis use an identical design rotor - without a screw!
If your rotor falls off because this screw is missing, you've got a much bigger problem ...
