Did Via write the KT133 / 266 / 333 driver? Did they write the KT400 driver?
Did SiS write the SiS AGP driver?
Did any company write any driver that's included in agpgart? I didn't think so.
Of course, the people that did write them depended on having open information to use, but as long as that's available (though maybe that's the problem) someone other than nVidia can write an agpgart sub-driver for nForce boards. I'm surprised it doesn't already exist -- though that could be because again, info on the chipset may not be open. If it isn't, then that is the problem that I think should be addressed here, not the fact that nVidia hasn't written an nForce AGP driver (except for in NvAgp). Once information is available, some kernel hacker with an nForce board will write the driver, it's just a matter of time.
I'd consider doing it myself next semester, schoolwork allowing, if I had an nForce board (for testing) and some info on it (for the initial driver write). But I don't have either.