Thanks all for your suggestions and replies. I ended drilling as much as the body as possible, until I could pull out the broken stem/valve itself. Once I got the valve and the two rubber gaskets (?) where the hot and cold enter, I was able to get the rest of the cartridge out fairly easy.
Sheesh...what a pain...
Anyway, it can be done, but as Mike said, BE VERY VERY careful you don't mar the brass valve body where the cartridge is mounted. It is soft and easily gouged or scratched.
Alls well that ends well.