Oh. Hehe. Well, yeah, it might be that the chip just doesn't support it. If the 3123 driver told you the same thing, then I'd say that the problem is the TNT2. That xvinfo run was with 3123 on a GF4 Ti4200.
You say you can't even use something like xgamma -gamma 3.0? The xgamma program uses the XFree86-VidModeExtension to do its thing normally (at least, according to the man page it does), and I don't see why the TNT2 wouldn't support VidMode...