I have to disagree with you here. Wikipedia is famous as the Encyclopedia "anyone can edit". If a random anon sees a page and tries to edit it, and cannot (while the main page still proclaims how everyone can edit), they are going to be dreadfully confused- lord knows enough are confused by the basic idea without adding on a second level of possible confusion. Perhaps two templates: the scary one for temporary semiprotection, and another, more discreet one for the more permanent ones?
Given how many web sites require registration these days, I don't feel there's a conflict between saying "anyone can edit" but you have to be registered to edit to certain articles. It's not like we're saying that only certain registered can edit, we would just be saying to you have to register to edit.
Sue Anne