I don't remember anybody posting it on here before, but i had a pic in a topic back on 3D Realms, where I went on about the Duke 3D box having no screen shots that were actually from that version of the game, or mine doesn't anyway. And the French text spoke of dynamic reflections.
If you were to try a particular version of Build from Ken Silverman's site, it would have a map that had the same mirror, so I figure build supported it by default (possibly). Makes you wonder why they pulled it out. Betas of things interest me a lot, and I can't help but type randomly like this in a thread that speaks of one :-)
That level is actually "WAR2.MAP" and not the level you play when running the game (p1.map I think it was, or something like that).
Edit: Attached is a shot from that version of build I was on about. Dunno what version, but the map is nsoal.map