Does anyone know a way to modify what objects server-side props collide with?

I have made a bunch of carryable props, but they're kind of awkward to use, since they keep bumping into the player. I've looked at everything I can think of but can't find any way to edit collisons.

It looks like the official props that the player can pick up get their collision groups modified by the game code, but there doesn't seem to be any affect it from within a map. Having a VScript method for modifying collision groups would be nice...