gsaurus did put together a level editor for SoR2, which may be found here: https://github.com/gsaurus/sor_pancakes/tree/feature/level_editor/Sor2_LevelEditor
I had leveraged it to change out enemies in my TMNT Remix and TMNT Final
hacks. It more or less demonstrates how limited VRAM is, as you can only swap an enemy for another one in the same scene (i.e., the same small part of the stage).
Can't recall if the Syndicate Wars hack adjusts for that.
The compression routine can be found around $30C0 in the SoR2 ROM. I never looked into whether it's specifically used for the level backgrounds, but I suspect it is, given that it's used for the enemy sprites, the end sequence images, and the title screen (I'm betting the player select portraits too). The level editor doesn't allow for changing the backgrounds, just like Pancake 2 doesn't allow for directly editing the compressed enemy sprites.