It would involve changes to the code in several places. Could mean more or could mean less, depending on whether the user interface should show that possibility or not, but it will always mean some changes to the existing code. I can't estimate the effort to do that.
Frankly speaking, I would set the rules with the moderators and exclude anyone who failed to abide by them.
It's not really possible without some serious coding. Keep in mind there's an option called Edited By on Date: under the forum's Feature Configurations. That'll show all mods and members who edited any topics and when they did so.
This account was hacked into by Image, a very honest guy as you all can see! Stealing people's passwords is his pasttime. Beware of this, before you register at his forums!