You're kidding right? Interfacing to some huge system that dwarfs the luametatex code base, creates a horrible dependency on what is fashion today (and replaced in a few years by what is then the fashion in graphics), adds plenty of runtime while in practice one can just create these graphics externally once and then include them.
If we talk 3d in mp we mean simple 3d for visualizing data and so. For artwork one should use programs made for that purpose (fwiw, going over the top in tikz runtime also makes little sense).
I see your point and also understand that rendering 3D is very time consuming compared to file access for sufficiently complex 3D scenes while for simple drawings it is faster to draw them rather than file access. -- Respect, Shiv Shankar Dayal