Shouldn't be impossible, I'll see what I can do!
The main idea is that you make the level in "layers" corresponding to different Z layers, then the engine puts them together when the room loads (moving everything to the first layer, and adapting their Z values accordingly). It's mainly there to make it easier to design 3D levels in GM's room editor and doesn't have any benefits other than that.
(There's a bunch of backgrounds that add a grid to the Room Editor; those backgrounds are used to figure out the size of the Z-slices, so it gets a bit more WYSIWYG than if you had to keep track of a "slice_width" variable or somesuch)
There's also the terrain/ceiling-terrain (which is probably what I named "Level Grid" originally... was a while since I touched the source code last!), which is essentially support for less rectangular ground, and which can be randomly generated. Its purpose is to make the ground in outdoors / cave areas look less like it's made out of cubes, but I couldn't figure out good ways to CONTROL it from within the editor, so it's a bit of an experimental feature. There's special objects that will generate (and draw) terrain if you place them in a room, TERRAINCONTROL and TERRAINCEILCONTROL, look into the init_terrain() script (which has a commented-out block of code that randomly generates terrain) and the "field" room for an example of how to use it.
The "layers that get stacked on top of each other" and polygonal terrain features aren't mutually exclusive, either, so this could be used for levels like a tower in the middle of a forest, or such. (In Dearelict, the trees are actually just very tall cylinders with a bark texture, players just ASSUMES they're trees that stretch so far into the darkness they can't see any of their leaves. Tricks like this can be useful to save CPU power for enemies and stuff like that, GM and 3D rendering isn't really a match made in heaven...)