allow subscribers to comment only

Thanks to plugins like viewlevel and manage roles I can control the role of posters and the level of posts in a quite effective way already. What I haven’t been able to do yet is give subscribers+ the possibility to comment and exclude all others below that level. At the moment you have to be logged in and have been allowed to post once to comment. But what if I want all users except subscribers to comment? The Manage roles pluging or the admin panel of WordPress 2.0 don’t offer that possibility yet….