what has worked in the past is to use Cola. I drink Diet Coke but use Pepsi for this task. Superstition perhaps. Take out BB, flip frame upside down and with intact seat post it was possible to pour cola into the BB and fill the bottom. Leave overnight and the post will fall out in the morning. With seat post cut like this, maybe a regular spray inside once an hour for 5 or 6 hours might be needed. The phosphoric acid in cola is a mild acid that will dissolve rust and gunk and free up the metal. Then flush with water and let the tubes dry and you should good to go. YMMV.