of a custom 8 string fretless Curbow, I think you would really enjoy the Dean as a fretless.
I will tell you that fretless octave 8s give new meaning to the phrase "intonation challenged," at least, at first.
One of the things that I really enjoy about my 8 is the wide spacing between the pairs of strings. It took some getting used to, but I can now play either course of strings seperately, and with it being fretless, I can get some crazy sounds just playing the octave strings (Robert Fripp meets sitar).
With the rockwood/rockwood lite neck, you'd be surprised at just how thin the 8 string neck can be. I think your wenge/purpleheart idea is great, and it should be almost as strong as the rockwood, so you shouldn't have to worry about playing a baseball bat neck.
As for the piezo question, I know that Billy Sheehan has a custom 8 string Attitude bass that has piezos in the bridge that he can set so only the octave strings are sent to the output of the bass. So, it is possible, but I imagine you'd have to search far and wide to find something for your bass.
I'm also about to buy a hollow Jazz body from Warmoth to build a piezo only bass with TI acousticore strings, so you guys have hit upon two of my current ideas in one thread (8 string fretless, and all piezo hollow basses)!