I still want the loading screen to show the players and minimap like a melee map loading screen, I just want to change the background. Is that possible? I can't seem to find the file to override though.
Open Reforged's CASC game archives with CASC Viewer and look for the texture files. The loading screen backgrounds are textures if I remember correctly. Once you've found the background texture, try to replace it at the same path but within your map's MPQ archive file (w3x).