I want to visibly indicate it on links / visits, for myself, so I can learn what the impact is and what might be worth avoiding. And how tightly bound use of it is - often these kinds of services are used/avoided by pockets that are somewhat closed (spam networks, indie webrings, etc), and I'd like to get a feel for where those pockets might be.

I assume there's some way to detect that mechanically, which is probably what you described. I just haven't yet seen anything that does this - hopefully it already exists, if not I should probably make it.