You could treat it as a readied action to make an unarmed disarm on the attacking opponent as he makes his attack. This would mean though that if your disarm failed you would still be hit.
That's the only thing I can think of. ahem! I have increased my knowledge of D&D 3.5 since this answer I posted so let me clarify;
When the player wants to use a whip, an NPC could use a readied action to catch the whip provided he had at least a round to be aware of such an attack been presented by said player.
"The Catch! you make an unarmed disarm attempt against the Players attack as the readied action goes off, a failed result will mean that the intended target is still hit with the whip and you would have missed your attempt."
If the check succeeds then you can make an opposed strength/or/reflex check/save to take the whip from the Player as you finish the disarm attempt.