1½ weeks passed, many things changed on the developer version, still available on dev.openstreetbrowser.org. I finally managed to implement talk-pages for categories (and later much more), you'll find them in the category info window. You can use WikiCreole markup to post your comments. In 'Recent Changes' (available under the links in the lower left) you can find the history of changes on categories and talk pages. Also the editing of categories should be much more robust, now you shouldn't involuntarily create a lot of duplicate categories any more.