UR probably figured this out already, but I was messing around with this and found that mid-air belts can also float pinball objects, floating pickups (food, fastfire, ammo), and bridges. The bridge toughness has to be set to an even number or you get a really weird animation bride that gives a divide by zero error if you stand on it.

You can make the falling boulders move before being triggered (weird effect). If the boulder is rolling through a set of conveyors like UR's tunnel of spikes, it will not hurt you, and will never stop moving, although it will not always roll in the direction of the conveyors. Obstructing the path of the rolling boulder will make it roll in the opposite direction without losing speed. Standing on top of the boulder has the effect of standing on a normal conveyor moving in whichever derection the floating conveyor event was set to.