If you go back in time, and revert to old iTunes data, it seems reasonable that you should be able to do this, then tell the iPhone before it syncs to revert to the back up (which there is an option for). Haven't tried it though.
Let us know if it works, if you try it.
Edit: I'm rather sure the iPhone is backed up in the regular iTunes folder, and not in the Application Support folder or anywhere else. I haven't cheched though, so you may want to rummage around and see which folder actually contains the iPhone backup.