Backdrop image files for the Millennium Environment are split into Left, Centre, and Right sections, so you’d need three planar primitives (aligned carefully) to display all of a given backdrop. It’s probably easier to load the M.E. with the desired image set, then go to the Surfaces tab, find the “TilingTerrain” material, and set its Opacity to zero.
Also, under the Scene tab, the “Terrain” bone within the Environment character seems to work for setting the ground plane to be hidden.
I tried this under DS3, since I had it open, getting ready for an overnight render. I’ll start that running, and try the M.E. in DS4 to see if these techniques still work there.
EDIT: Yep, in my DS 4.0 (not ready to risk 4.5 yet) the Scene tab and the Surfaces tab do the same jobs as outlined above. Either select the “Terrain” bone inside the M.E. character and click off its visibility eye icon, or go into surface selection mode, pick the ground, and set the “TilingTerrain” material to zero Opacity.