Add `rel=ugc` (optional?)

I’m hosting a public/open registration writefreely instance.
Most of the registrations on my instance are advertising/linkfarms/etc.

As I don’t like this, I would love for writefreely to add a rel="ugc" tag to links for search engines know how to weight those links.

(Yes, I know rel= nofollow is already in use, but ugc actually defines the content/links better IMO - and it’s also possible to have both)