So the new mesh system is almost fully integrated (as much as the old one at least), including the new LOD system. As I said this new system is easier to use, in fact today I exported a simple model from Blender with two LOD levels:
I only ever tested the old LOD system with simple hand-made meshes, it would have taken a lot more work to get a mesh into it from a modeling program, so I'm happy with this. I'm still considering how (and whether) to implement smooth blending between the LODs (which was already working to a decent degree in the old system), but I think I'll leave that until later. On the plus side, this new system should be more efficient, is more modular, now supports binary files, and is much faster to load.