Are reverse lens mounts only for use with prime lenses? I was wondering if I could use my zoom lens with a reverse mount.
From a mechanical viewpoint, you can reverse a zoom just as well as a prime -- you're using the lens' filter ring, so the type of lens makes little difference.
From an optical viewpoint, at least when I've tried it the results didn't impress me much. In fairness, however, reversing lenses tends to be a bit hit or miss, so I might have just had rather bad luck.
Reverse mount are intended for the filter thread of a lens. With that in mind, it is clear that you can mount a zoom lens as well as a prime, reversed on the mount (as long as it has the matching filter diameter, of course). Note that there are rings that let you mount a lens onto the body mount (say EF mount for Canon), and rings that will let you mount a reversed lens to the filter ring of a straight mounted lens.