I've been thinking that one of those new electric nailhammers might work well to vibrate a stem and seatpost loose, without harming the frameset or fork, plus you can direct the vibration and impact wherever you need it.......sort of a poor man's impact tool.....with no need for compressed air....... I think Ryobi and B&D makes them and can be checked out at your nearest big box hardware stores.

Chombi