On the whole I would be against an idea like this, mainly because I don't think it would be worth the extra resources it would consume.

Okay, I know it wouldn't use much, but it wouldn't give much functionality in return either. It would have to be smart enough not to print multiple rules if there weren't users of a certain level logged in.

i.e. It should display:-

Level 7 users
-------------
Level 3 users
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Level 1 users

not

Level 7 users
-------------
-------------
-------------
-------------
Level 3 users
-------------
-------------
Level 1 users

Also, it would have to be an HTML <HR> rather than an XHTML <HR /> tags as the rest of the site is HTML 4.0 compliant (according to the DTD anyway).

Maybe on a smaller, less busy site, without a chat box etc. it would work, but the default layout is already very crowded.

That's just my personal opinion, feel free to ignore it.