My take is that first of all I would make them "lightweight" stone, i.e. they do not get heavier when dead. I would use the rules for squeezing to get past them, which means medium or small creatures move at half-speed through the one square (with normal squeezing modifiers at the time). The horses, however, would require an escape artist check to pass; which probably means they will not do it easily.
I would not have any other modifiers to fight while in the square with the statue (squeezing is quite enough). Any normal balance checks you have while on the bridge should already apply. Freeing yourself from buried rubble is a DC 25 strength check, so I would use something similar, i.e. at your upper end (so DC 23) to actually lift the statue and toss it over without risking damage to the side. Pushing it over should be relatively easy, say DC 16, and give the rope a DC 20 reflex save to avoid 1d6 damage from tearing (each side has 8 hp and 0 hardness). It's not likely to tear the side because I'm sure the heroes will alternate sides and the draconians will probably not keep dying in the same spot.
Good luck, and have fun!