Thanks a lot for the thorough response.
Yeah I imagined it wouldn't be able to do much relative to blender materials, but I also imagined backface culling was about as regular as it gets to the point of expecting that be the default, so was surprised when I couldn't get it.
Yeah that CPU culling doesn't sound like a possible solution to be trading that much performance away. When you say force enabling of back face culling causes issues with non-3DObject objects, does that mean if your project just uses 3DObject's then, this could be an option? Or does your followup comment mean there'd also be further issues introduced if trying to take advantage of 3d located particle sprites too like in your 3DMeshPoint example?
I've got a blender, gltf, and minimal project (the model in it) here. Also a minor and not so minor thing, in the editor the 3d model textures are softened-but it does respect the nearest/point setting when you play the scene so it's not a big problem if that can't be resolved. However also, maybe a limitation of C3, possibly a rendering option though, but the softening of the texture also softens the alpha, and this causes that to pierce through other object as seen here (the left side has separated objects where as the right sides are merged so you can see it pierces per object).
Would be awesome if you could make some advancements with this as backface culling is an especially valuable technique when having to work with a more restrictively performant 3D environment, but I can understand if it might not be possible. Appreciate you're still willing to try and tackle it/have already made attempts.