In case anyone is interested, this creator built a remake of Portal for the N64, uploading a really cool set of videos describing the work that went into building it.
He's since stopped to work on his own IP, I believe that the issue was that Valve couldn't allow it because they'd never get Nintendo to agree to it. Something along those lines, anyway.
The same guy, James Lambert, also implemented texture streaming (which would not be invented until two console generations later) in an N64 demo. The textures look uncharacteristically high res: https://youtube.com/watch?v=Sf036fO-ZUk
He's since stopped to work on his own IP, I believe that the issue was that Valve couldn't allow it because they'd never get Nintendo to agree to it. Something along those lines, anyway.