The idea of an expander holding a seatpost on has been around in BMX/freestyle for a while. The ones I've seen use one bolt to hold the pivotal seat onto the post, which also threads into the expander and pulls it up into the post, which has a pair of slits like a stem. The posts used are typically around an inch long, and seem to be referred to as "nub posts". As such, it's clearly possible for a such a design to work, althoug BMXers don't really sit on their saddles, and the post is too short to exert much leverage, so the fitting doesn't have to resist as much force as it would on a road bike. You'd certainly want a lot more post inside the frame, to start with.