It seems that I wrote an plugin to give moderators the ability to approve users avatars some time ago for some reason (probably a client, I can't recall just now).
https://github.com/Sama34/OUGC-Avatar-Aproval
If someone wants to give it a go, go ahead.
You should also make a plugin which does the same thing but for signatures
That is what I thought when finding this within my repositories, however I can't remember the reason for coding this so there is no really motivation to code the same for signatures.
Understandable. But if you're ever bored and looking for something to do you should make that! Unlikely you'll ever not be busy though
I have an 30+ TODO/wish list growing since back 2012 so yeah, you can guess I don't have the time I would desire.. there is one to allow users to upload one image to use in there signatures that I want to code the most, just need to find the time.
Anyways, your suggestion is already in the list