A messy, short-term ("emergency" "get you going again") solution is to undo the hose (or hard line) into the cylinder and bleed as best you can. I've done this successfully when there was no other option, i.e., 10:30 on a Sunday evening!
As noted, you might be able to save the wheel cylinder with a left-hand drill bit or an "Easy-out" extractor. It can't hurt to disassemble the cylinder, put a propane torch on the area around the bleed screw for a minute or two, then try the extractor; this assumes, among other things, that you can easily obtain a proper replacement bleed screw. Of course, unless the cylinder is perfect otherwise, your best bet likely is to replace it.