So long as you're confident that you're turning (or rather, applying force) in the correct direction, it sounds like your issue is to apply more force without chance of damaging the frame. Best option is, as you say above, to take the frame home and stick the fixed cup in the vise, and you can use the whole frame as a lever to try to twist the fixed up off. This is much better than hammering on anything.
It may be worth setting the frame on its side with fixed cup down, and putting some penetrating oil on the cup's interface with the threads of the BB (apply from inside the BB, reaching through from the non-drive-side) and let that sit overnight first.