I just took a look at that area with a Longplay video and yeah, I hate those static non-scrolling backgrounds the most. You *can* sometimes turn them into a repeating pattern with some clever (or at least, acceptable) tile editing if you're not too fixated on absolute graphical authenticity.
Looking at your map of the area dark_lord_zagato, I think it was the right decision to retain the background for those horizontal areas. I think you could assemble something that works for the rooms where you left it blank but you'd have to be creative for those large rooms with a sloped ground.
