My main worry would be putting a ding in the seat tube - maybe tape some flat bits of wood to the sides of the seat tube so there is no single pressure point. I wouldn't worry about the general amount of force going into the seat tube. Framebuilders do a lot worse when they're aligning frames and bike frames come under a lot of force in normal use so they're pretty strong.
My main worry would be putting a ding in the seat tube - maybe tape some flat bits of wood to the sides of the seat tube so there is no single pressure point. I wouldn't worry about the general amount of force going into the seat tube. Framebuilders do a lot worse when they're aligning frames and bike frames come under a lot of force in normal use so they're pretty strong.