1. Thread a heavy duty bike tube into it with a strong nylon cord tied to it.
2. Use a pump with a straight hose connected to the Schrader valve (12v auto pumps often have straight screw on hoses).
3. Once entire tube is pushed into seatpost, start the pump for long enough to assure the tube is good and stuck.
4. Pull the nylon cord.

Never tried this but it seems possible.

OR jam one of those plumbing de-cloggers that attaches to a garden hose into seat post (if narrow enough is made). Stand back. Turn on water. I think the bike tube method has a higher likelihood of working out though.