If any player in my campaign asked to do this (and I really wish they would, they've done exactly one stunt in the campaign so far...) I'd rule that they would first need to grab and then make a dex vs reflex attack. Since a straight up dex attack won't have any bonuses from magic weapons/implements, this will make it quite hard to pull off as the levels go up, so I might even allow them to slide the target one square. What makes it even harder is that grab is a strength check, so the PC is required to first hit with a strength attack and then a dex attack, without benefits of weapon bonuses.
I might add a size bonus to defense, maybe +2 or +4, to avoid sillyness with large monsters with low ref. Or maybe require the trip to be vs fort?
I might add a size bonus to defense, maybe +2 or +4, to avoid sillyness with large monsters with low ref. Or maybe require the trip to be vs fort?