Being careless with a screwdriver (after far, far too much time being conscientious with this project....), I managed to create two 2mm dents, from the inside, of my new-to-me Trek 613 with a massively seized seatpost. Pics are below, with a bonus shot of the mangled and still *mostly* seized seatpost.
Trying to bright-side things, I've been considering learning some serious frame repair anyways. So, what's my repair here? What do I need to do it? Can I get away with using MAPP gas? If I need to replace the entire tube, where does one pick up only one Reynolds 531 tube or equivalent?
If you could somehow get the remainder of the seat post out, you could probably put a new one in and beat the dents back in with a wooden punch.
The NDS seat stay cap looks funny, hopefully it's still attached.
If you do replace the tube, just get a modern tube. You can find NOS sets of 531, but it doesn't seem like a good investment. To replace the tube, cut a 3" section out of the middle of the tube. Optionally thin the tubing left in the lug. Heat the tubing from the inside, remove. Repeat on the other end
This is why I don't recommend mechanical destruction of stuck seat posts. Did you try heat?