Originally Posted by
MudPie
Wow, this sounds like a neat challenge! People who work in the well drilling field have various ideas and fishing tools to extract broken vertical pipe to recover.
My idea involves a tool that acts like the attached "sprinkler pipe riser remover". The idea would be to use this type of tool to turn and break the seat post free, assuming there is some corrosion or debris jamming it in the seat post. Once freed, you'd still have to remove the seat post, like using the methods described above.
https://www.lowes.com/pd/Orbit-Riser-Extractor/3420774
Along those lines you could probably drill & tap the handle of an internal pipe wrench for some all thread as well.
https://www.lowes.com/pd/Kobalt-NA-i...E&gclsrc=aw.ds