I have been in need of the same functionality. I am exporting animations from Unreal then importing them into my game engine. However, the game engine I am using can not import animation .fbx files that have no preview, and the preview has to be under 10k triangles. So without being able to export a specific LOD of a skeleton rigged mesh, or without being able to select the LOD used as the preview mesh, I am forced to try and export from Unreal, import into a 3rd parts, decimate, then export from 3rd party, and import into my game engine. Not only is this a LOT more tedious, but some 3rd party, like Blender cause weird things to happen. If Blender would act decent, I would just export a character mesh from Unreal, load it up into Blender, selecton only a low LOD, then re-import into Unreal, to use as the preview mesh, however, once I export from Blender, and try to re-Import into Unreal, it says the Skeleton Hierarchy has been modified, and it gets all messed up.