Chris,
You may not want to, but I have done this on Caterpillar engines...square head pipe plug right? Weld a nut (attach ground directly to the nut) to the plug and let completely cool, the shrink effect and heat usually breaks the seal/sealant and allows for easy removal. If you are worried about welding onto your engine, then use the smallest torch welding tip available and heat up the plug, you might want to make a mask out of sheet metal for protection. Let cool and try that, then when the end of the plug strips or breaks off, refer back to the first method. Experience taught me to just go by method #1 and save the frustration of having to weld onto a plug that has broken flush with the block, and having to do it twice. When they break off flush, then you have to get the TIG rig out, build up an area in the middle and then weld the nut on. Drilling and using lefthand threaded easy-outs really aren't that great of an option because the they tend to expand the fitting tighter plus the risk of shavings and debris in the engine.
Good luck to ya!