Actually, there are two versions of Yeti3D. One is public and free, the other commercial and private. The thing is, the free one doesn't support Quake2 models whilst the commercial one does, so it'd be much much easier to make a game in the commercial one.
That said, I don't imagine anyone would complain if you worked the model-compatibility out yourself when trying to use the engine. Darn big task though, methinks, else no-one would use the commercial one!
Last news I heard on it, ThunderZ was adding sloped surfaces to both free and commercial engine ports. That was quite a while back. No idea since then...