From the rules:
"If your opponent is holding a light weapon"
Natural weapons are considered light for the purpose of Weapon Finesse (feat text), as well as by the text in Power Attack (where they're specifically excluded from the no-light-weapons clause). However, you aren't considered to be holding a natural weapon. It's likely a gray area, I could see reasons to go either way, but I don't think there's a clear reason to say that you could do that.