If a background sprite is assigned to the splash screen, but the splash screen is then disabled in the player settings(so it can be drawn using a script instead) then the background will not be included in a player build.

1) Assign a sprite to the splash screen background
2) Disable the splash screen in the player settings
3) Build a standalone player
4) Draw the splash screen using the scripting api in a scene
5) A blank background is shown instead of the sprite