IANAWD, but couldn't you have several opaque layers and modify their
z-indices?
I actually did some messing with the z-index property a bunch yesterday, for a completely different reason, and it was a big pain in the ass. The objects in question, for my particular layout design, got rather messed up and didn't play and wrap right when I started using the z-index property.
I agree that the approach would work, but what I'm saying is that in my particular case, this probably isn't a workable solution.