Nope, all level geometry artwork (except a couple textures in a couple maps, which come from the standard wad or old demos) is custom designed.

What I did was model each unique piece of hallway - i.e. straight section, corner, T-junction, etc - using level blocks built as map entities, and set up each one so that when you walk on it, it fires a trigger event. This event sets the visibility of all the other hall sections (and any bipeds and weapons resting on them) based on whether or not they can be seen from the section where the player stands. It's not too efficient (visibility has to be manually calculated by the map designer and gets less effective in multiplayer), but the game certainly runs faster than it would without it.

I would've just placed the halls as normal blocks and used the built-in BSP visibility calculations, but the build time for that method took much longer than I had patience for.

Edit: The two campaign maps handle visibility in two different ways. In the first one, every transition between map entities is clearly marked with three doors, or two doors and an invisible trigger. The first door makes only the first hall visible. The second door, upon being triggered, displays both halls. The third door displays only the second hall.
In the second map, I used the much more refined and reliable method of using small sections which trigger as you walk on them. No more doors to worry about.

Oh, about the installer, I will be sure to drop it for the next online release. That was set up for convenience to a local audience that was receiving the game on CD.