Thanks for all the suggestions and ideas... there is not much of a lip for a set of vice grips to bite into on the outside, but I'm thinking that I'll try heat, with a slightly oversized torx bit pounded in (which should also give the "shock" effect to the threads), followed by an impact hammer and chisel at an angle to see if I can at least get it started. There should be enough of a lip for that.
If that all fails, maybe a bolt head welded onto the plug?
Anyone have success with any extraction products along the lines of this:
http://www.use-enco.com/CGI/INSRIT?P...FdUXHwodcGkMBA
Scott