I did give Hilbert curves a try at one point, what made me go with z-order instead is that there's an elegant way to convert between coordinates and z-order quads (which I'll describe in a later post) and I couldn't find something similar for Hilbert quads.

That's an exaggeration. The Hilbert curve calculations are also done using bit flipping. They are harder to understand, but are not that cpu-intensive.