Well, an early experiment with both the token [SOIL] and [DEEP_SURFACE] created some fun. My black dirt did, indeed, form the deep surface as intended... but it also formed two of the four soil layers on the initial test site. The fun bit was that it had included clusters of Slade as intended (with that having embedded veins of Obdurium), but only on the top surface layers. The inclusions didn't appear buried hell-- that was uniform.
I'm going to have to test various methods of excluding it from the surface without removing SOIL, and adding plants to it... then see if specifying multiple DEEP_SURFACEs will share the space, or each one take a biome of its own.
EDIT (11:15, CDT):
I'm keeping a record of what I learn here, on the off chance that anybody finds my experiments useful, or can provide me with some insight. An experiment was run with the "deepsoil" material having been given the tokens [SOIL][DEEP_SURFACE][SPECIAL]. It once again showed up at both the top and bottom of the map, with inclusions of Slade (and my Obdurium, which I'm still working on balancing) only in the top layer. I revealed Hell and let the game run for about ten minutes... none of the grass I created was growing down there. Tried muddying it by creating a river source down there and letting it muddy things up, then drying it out and letting the game run a while longer... Still no growth.
Next experiment: Remove the [SOIL] token, make it vaporize on mining instead. Soil obviously isn't making things grow on it anyway, so I might as well take it out. Guess I'm going to have to try to find a way to muddy the HFS while the player's busy above. Generated a world with three [DEEP_SURFACE] materials to see if I can get some diversity down there that way, with plants on the back-burner until I figure out how to muddy the floor. The first embark in this world has only Deepsoil down there, no Slade or Obdurium despite all three having the [DEEP_SURFACE] token. Abandoned, looking for a site where I can find a meeting of multiple biomes... Site found, and Hell revealed. Once again, uniform. At this point, I'm stuck. I don't know what to do to break the uniformity of the bottom layers! Does anybody have advice?