Well, not really, but it’s a reasonable fake in the Quake engine. It’s a pretty simple trick that other editors and exporters have used but I just added it to mine. What I do is have a special tag I place on meshes inside of Cheetah (in this case, just putting a “*” in front of its name). When my exporter sees that it treats every triangle of the mesh in question as a separate Quake brush and it adds polygons behind it to make it a complete shape. This leads to some exciting possibilities. Look at this mesh, for example:

It looks like I’m building a cliff or hill or something but that sky texture on top is out of place. Now suppose I delete the bottom polygons and open this thing up. If we then flip the normals on these polygons and look up from the bottom it looks like a canopy of cliff faces with sky above. Like this:

And here it is inside of Quake with some extra brushes added and proper lighting:

This opens up a lot of cool possibilities and really limits how much brush drudgery I have to deal with inside of Cheetah. I can rely on Cheetah’s strength in modeling shapes and let the exporter turn it into something Quake friendly. Love it!
Here’s another use of this technique, adding some rock details into the ceiling and some dirt piling up on the floor:

I foresee much fun with this!