Most dealers should be willing to rent a bike to you, although it would help if you or someone you know speaks German well enough to explain what you want. You may need a translation of your license. I've done it once in Nurnberg, and my dad has done it a few times in both Nurnberg and Munich. The rates weren't bad when I did it, but that was in 2004. You may get some resistance since you won't have a German address, mostly because they don't want to be liable if you get a traffic camera ticket or something.
Stephen Laffoon
WERA #135