Reddit Enhancement Suite (RES) has a user tagging feature that I used a lot. As you described, you could give a user a custom tag that would show next to their username. There was a lot of customization available.

One guy by the name of shittymorph would occasionally get me with his troll posts so I gave him a custom tag and made it neon green so I would immediately recognize his posts when I was scrolling.

https://www.reddit.com/r/Enhancement/wiki/index/users/user_tagger/