Heat and hammer looks like the only option now. Use a sharp cold chisel and try to get it hooked on the remains of the teeth or get a sharp punch and make a good indent before you try to tap round. You need to hold the frame still, in a well padded vice or on a sandbag.
And you still have the option to weld a small lug onto the cup as a last resort.
It will come out, it just may take some bloody mindedness.