I know this could easily be done with Unity, but it shouldn't need an entire game engine to do this. I would rather not make the visitors install Unity just to look at a model. This is nothing against Unity (I love Unity).
I have a feeling this isn't easy or even plausible, but it doesn't hurt to ask.
Thanks.
Visitors don't install Unity, they install the Unity webplayer plugin, which is quite small (and helps increase the Unity webplayer install base). It only has to be done once, plus it's handled automatically. So there's no real reason to avoid using Unity...the whole point of the webplayer is to allow good 3D on the web.