You could make the fog part of the level's background on the high plane, like SLZ or, at least, that's what I think SLZ does. It could part of the foreground on the high plane. I'll have to look at it later. Surprisingly, after a year of hacking, I never edited any levels past SYZ. I guess you could also make it a palette cycle (Like what Pacca did for her port of Mania OOZ's gimmick) But probably wouldn't be the exact effect you want. Well, hope some of those tips were useful! Nice tree, by the way.