For an opponent to provoke an AoO you first have to threaten that opponent. Logic would seem to dictate that then you are limited by what attacks you can make that satisfy that first requirement, meaning, if you don't threaten with an unarmed attack, you can't grapple on an AoO, although you could still hit with your sword (which you do threaten with), could still try and sunder (with your sword) (which would provoke an AoO if you didn't have I.S.), etc. At least in my game, this is how it works. If you want to unarmed trip an opponent on an AoO, you first have to threaten an opponent with your unamred attack for tripping (be it through I.T. or I.U.S.).