Christoph,
if you feel I've critizised your recommendations, this is not the case (the "it's interesting" part to me sounded like I had broken a rule not to talk about courses you haven't played ...). My points were just supposed to be supplementary comments.
Regarding the Duisburg area Krefeld just came to my mind, because from what I've heard and read it is one of the courses I want to see, while Niederrheinischer GC is not an that list (at least yet, maybe I've just missed it). Exactly because I have not played it, I haven't in any way recommended it, but said "you may want to consider" giving Brian another idea to research further and inviting comments on it from people more knowledgeable about it - like you appear to be. And honestly I was not aware that Duisburg-Hubbelrath is actually the same distance as Duisburg-Krefeld - I'm not too familiar with the region and felt Krefeld was nearer and gladly stand corrected (while the drive may still be a little quicker?). I certainly concur that Refrath is a more interesting place than Krefeld if that isn't too far off - Hubbelrath probably as well, I just thought it was an interesting course a little nearer to Duisburg - and was mistaken as to the distance.
As far as Beuerberg is concerned, it is without a doubt a very good colf course (especially the routing) and if you want to study Donald Harradine in particular it may even be a must. However it is pretty far off and a newer design (where Brian said he'd be more interested in older courses). While you are right that the atmosphere does not hurt the quality of the design, I feel it somehow flaws the overall experience of being there, so I - just as I said - would not consider it a must, while I certainly would not advise against it, I would just not rank it that high given the specific circumstances.
Regarding Garmisch that's not a small detail, but a major mistake on my part: I was told by a friend who is very fond of the course (but that of course doen't mean he really knows about the club's history) that the course in Oberau was in fact the old one (and had 18 holes from the beginning), which was used by the US Army and later returned to the club and the smaller 9 hole course was built as a replacement (while the Army was using the original one). I read about Garmisch in the DGV's publication for their centennial and was not really enlightened by that about Garmisch's history ... so I learned a lot more here ...