I'm a big fan of any sort of long duration vibration to help penetrating oil do it's job. Let parts sit on (or with stem), lean against a job-site compressor, or a generator, for hours. while ensuring there is a constant pool of oil sitting above seized section. The combination of gravity and thousands of cycles of vibrations usually leads to successful outcomes. Took 2 days, once.

I have one abject failure. A stainless steel speedplay pedal axle stuck inside a Dura-Ace 7402 crank arm. I don't think one iota of oil has ever gotten even the teeniest bit of penetration into those threads. I have even tried heating the arm with a torch, and sticking in the fridge to try to get some gap to let the oil in.