I'd actually be worried about denting the seat tube. I've seen this method used, and seen enough problems not to recommend it.

There is another way I forgot to mention. Get a sturdy workbench or table top and lay the bike on its side with the bottom bracket on the table but at the edge. Be sure the head tube is on the table as well. At this point, one person can hold the head tube down and another person can push down on a chain stay. This is reasonably safe if you are careful.