This might be an ok idea, but we haven't done it for a few reasons. The main one is that it's easily bypassed. If it were added I wouldn't want to add the same feature to my mod (limitations are annoying). Others would figure out how to remove it too in their own mods. Also, really only direct copies are the problem. Sometimes people take saves and make them better and / or fix bugs. Also sometimes people come up with good ideas and want to use one part in their own saves with credit, this would just make that harder since you would either have to find a free to copy save, or make a part that's already been done before yourself. You could instead just nicely report any copies of your own saves that you find, and a mod will unpublish / delete it. I really doubt I could get Simon or any other moderators to support this (especially Catelite who said copying wasn't against the rules at one point), and I don't want to anyway.
This is a really good idea, and a great way to solve the problem of others copying the save.