How far in is the seatpost?

You can try an auto dent puller threaded through the mounting bracket if you can get a good enough purchase on the seatpost.

Try using a penetrating lubricant like Break Away.
you can also try some heat so long as you dont go hot enough to melt any of the brazing material.
Conversly, try cooling the aluminum seatpost with some dry ice to get it to shrink, or use a combination of cooling the aluminum and heating the steel.