I looked into this at one point (I juggle occasionally, and that though had popped in my head).
They're out there, and with a lathe, they aren't terribly hard to make.
You'd definitely want a hardwood. Something that's dense enough would likely wiegh close enough to acrylic to work ok.
One downside, they aren't transparent, or even translucent. I really wanted to backlight mine so that kinda ruined the idea.
OTOH, if you could fashion a translucent "ring" to replace the standard black ring, and light +that+ a wooden trackball could be fantastic.