Cool plugin! Saved me from having to write this same thing myself, which is always a plus!
We want to limit sigs based on user group membership; specifically we want subscribers to be able to have larger sig graphics than non-subscribers. This presents a problem for your plugin, however, because it only checks at the time the user edits their sig. I understand the reasons why this is done, and agree with them. My question is, have you got any idea what vBulletin hook I could use to check the user's sig when their subscription lapses? What I'd like to do is simply remove the graphic if they are no longer authorized to use it.