Thanks for the responses guys. Looks like this one might be out of the realm of the reasonably possible (for me).
What I had in mind was to create a backdrop which included a picture of the red and white moons (to remind players in a kind of cool, subtlish, way that this is a Dragonlance mod). But in FRUA it is not possible to have different backdrops depending which direction the party is facing.
So I thought of creating a "wall", mostly blank but with a picture of the moons, with the intention of using it on one side (say the west) of each cell that does not otherwise have a western wall, so that whenever the player was facing west it would appear that the backdrop included the moons (but not when facing n, s or e). I think this might have been too hard anyway, because where I use low walls I would need multiple versions of those walls as well. I would run out of wall slots.)
I think it would probably be somewhat annoying / too unrealistic to have the moons showing in the sky as part of the backdrop (in the same place in the sky) in all directions. So I am probably going to elect to have no moons showing.