My last resort is to shock it heating it with a torch gun then squirting it with WD-40, Binky, especially when stems want to be saved {works everytime!

I have an issue with a Miyata frame though atm that got given to me, he started sanding the frame down but couldn't get the seat post out, cut it flush to the seat tube, lol, dang
drilled a hole on the back where you can, fed a wired rope through, and hitched it up to a tree and my towball, broke the damn wire so upgraded to a stronger one then broke the rope on the towball, upgraded that to a chain then broke that other wire

i may need to use your method or dissolve it with caustic soda