Zbrush is more about 3d model elaboration (painting and sculpturing). In what I am suggesting there are no model at all: just "aerial" brush strokes that can get "stored" in a layer.
Such brush strokes are "stored" in layer and are used to "paint" directly the image in the eyes of the viewer (as the individual who watch it). Zbrush rely mostly on 3d models; thus "the viewer" is indirectly affected by the show (giving him/her more with the possibility of move the camera as much they want)
Zbrush at the base was really oriented as a 2.5D program , with a canvas where you can throw pixel with a Z depht infos ( named the Pixol ; from the editor name Pixologic ) . Zbrush could create 2.5 painting who got a 3D look by using 3D model as painting brush , and with the ability to paint texture and material on the top.
What makes Zbrush famous in the 3D modeling world , was the possibility to customize the 'Brush' tip ; basically a 'real' 3D object and not only a 2.5D. The creator did a sort of sculpting mode very fast , to can sculpt a brush , then bake it on the Z canvas ; and paint the shader. The popularity of this feature growed when Zbrush could import and export a brush ; and most of artist started to abuse the Zbrush brush system to sculpt. Now most of poeple think Zbrush = Sculpt 3D object , and the developpement of the software goes into this direction ...
I explain all of this , because I think your "pseudo 3D layer" concept is exactly what Zbrush is done at the origin ; the 2.5D with a depht pixel ( without having 3D object as brush ).
(ps: the video Pyo posted is about a 'brush creation'. Watch here http://www.youtube.com/watch?v=QxjNwoGlkFY
, to see an artist using 3D model as brush and bake them on a Z canvas, and play with light , paint material....etc.... he use Zbrush as the program were designed at the base )