Some transparent blocks (e.g. slabs, stairs) can't have a transparent texture.

Some transparent blocks, like slabs, stairs, etc, do not induce a cull on other blocks, yet they can't have a transparent texture. This can be annoying for creating more detailed block models without having to have a lot of polygons.

At full blocks (like cobblestone) I understand that the textures can't be transparent. At slabs, however, it shouldn't matter if it's transparent or not, because it doesn't induce cull (as far as I know), and thereby can't produce x-ray resourcepacks.

(Information of the screenshots)
What I'm trying to get here is the effect of a bump-map. I made a central element, which is completely opaque, and a box around it, with transparency in between the rocks (the black parts). This way, it would look as if the rocks were coming "out of" the block, with only two polygons per side of the block.