While browsing the articles on MacWEEK, I stumbled on this article about developer support for the G4's AltiVec unit, aka. the Velocity Engine. Lo and behold, Bungie Software is in that list.

Later in the article can be found some notes regarding OpenGL on AltiVec, though how much Halo benefits from this will depend on how Bungie supports OpenGL. Material for the FAQ, that's for sure...