I recently got a alloy seatpost out of a steel frame using caustic soda. It corrodes the aluminium but not the steel. It does damage the paintwork but if ur careful (keep it on the inside of the frame) there will be no damage.
I'm toying with doing this too. Living in an apartment at present though, and fancy getting my deposit back. New house (so long as the bank gives me money) needs a tonne of work so I can wait til I'm there to try that or the saw method.
I recently got a alloy seatpost out of a steel frame using caustic soda. It corrodes the aluminium but not the steel. It does damage the paintwork but if ur careful (keep it on the inside of the frame) there will be no damage.