I followed up with my tried-and-true Sawsall method after narrowing the height of my long Diablo demolition blade on the bench grinder to fit inside of the seatpost ID.
Trick here is to probe the slot with a sharp pointer of some sort to check for any remaining aluminum bridges along the cut slot in the seatpost. Even the tiniest of bridges will prevent easy turning of the seatpost stub!
The blade totally glances off of the relatively hard Reynolds 531 seat tube, so no danger of cutting into the seat tube.
