Can I use a Fujinon s20x6.4brm-sd lens (currently residing on a DV500) on the JVC HD100, and if so, can anybody say what is the quality of the resulting image?

The lens will have been designed for a sensor having a much lower pixel density than the HD100, so it will probably be lacking in sharpness. Your wide end will also not be very wide at all.

If I'm not mistaken, that's the lens we used shooting with the JVCs last years CMSA series. So to the clarity, no, threy're not true HD quality which isn't such a bad thing depending on the shootr's experience and the subject matter. And the DOF was great on them. It really all depends on what you're shooting and the distance. We were shooting deep, fast-moving sports figures and they were exemplary but up close in the CC booth was where the HD fell apart.. Hope this helps.

The DV500 is a 1/2" chip camera (I have one), the pro-hd series is 1/3". I wouldn't expect it to fit without an adapter (the JVC adapter), and in any case, there will certainly be an adjustment of focal length towards telephoto.
You will just be using the center of the lens, which helps somewhat, and you'll definitely have the option of a very long lens. If that is an essential shot, it's probably worth it.