On the telecon of 2012/04/25 Bert Bos argued against using an @rule for region styling. He proposed a combination of pseudo-elements and the new hierarchies specification instead. I'll add a link to the minutes once they are posted.
The current state of the specification on region styling is a result of discussion on www-style here: http://lists.w3.org/Archives/Public/www-style/2012Mar/0490.html
The telcon minutes are here: http://lists.w3.org/Archives/Public/www-style/2012Apr/0757.html
Since then, we have considered relaxing the rules on what can follow a pseudos in a selector. If those rules change then there may be a way to use pseudos for region styling that is preferable to the @rules.
Reconsidered, and changed to a pseudo-element: