HL uses a modified form of the Quake I engine last time I checked.
Also, you're completely wrong about HL being limited to bipeds. You can do whatever the heck you want to. There's been a racing mod and all.
Years back before I "saw the light" while I was stuck in that awful mess that is rowboat-tech, I had a New Gen / MOSPEADA project started for HL. I had the motorcyle mode of the ride armor semi-functioning, but the HL code is just a mess and kept frustrating the heck out of me. In addition, I had discovered the true originals, mainly Macross, and said "screw this americanized crap". Add to that a lack of support from basically anyone around that time, and you pretty much see why the project was shut down.
I plan on eventually doing another first/third person MOSPEADA game, but not for a long time.
Oh, and BTW - my RT/MOSPEADA mod for HL was called "Invasion" (Originally "RT: Invasion" but then changed) - so it looks like that new console rt game stole my title...
Actually I think we talked back then. I had a RT:New Gen/Mospeada mod for HL and a website up and I think we emailed each other a few times.
LOL. We both said screw it.