Actually just got layered movement working! So that build is up whoo.
I spent a few days trying to do it some weird way... which was defining a bunch of "bottom planes" for each object and automatically sorting the layers. That did not work and my brain hurts.
But the way that does work it easy! I just check for overlapping objects and stick her in the right layer. Whoo. sob. That took like 20 minutes compared to my first attempt.