I have done it.
Mix up caustic soda with water as described on the bottle to the strongest mix (mine said for paint stripping).
Pour caustic soda into frame through bottle cage bolts on seat tube with frame upside down using a small piece of aquarium hose and a small funnel.
Leave for 3 days I did try each day. After 3 days you could just see that liquid was seeping down the seat post.
Remove saddle and put end of seat post in a vice and twist.
No damage to frame, seat post is obviously no use afterwards.
Make sure you collect and clean off any spilt caustic soda it killed a big patch of my lawn even though I washed it off almost instantly with lots of water.