I'll probably rule that such maneuvers are very difficult to pull off reliably in combat and declare them something like Str versus Fort or Dex versus Reflex attacks with large penalties (-5 or 10 or something). This should make it possible, but unattractive as a combat tactic against level-appropriate opponents.
I've played with Disarm and Trip focussed PCs and I have to admit I've found them pretty irritating in play. I much prefer it when both sides get to show off their most menacing powers, as opposed to getting tripped and disarmed in the first round and looking like a dope. Tripping and disarming in most fantasy action seems to happen to mooks, surprised characters and worn-down opponents, not to the Dark Lord at the climax of the adventure. It's a bit easier to take when it's a special trick that the PC can only pull off occasionally, rather than every dang round.