Correct - not possible for the KMS reasons above.
Nvidia have only just gotten around to supporting XRandR this year, despite the feature existing since 2001. That's pretty awful turn around.
Linus Torvalds (original author, and head of the Linux kernel project) criticised Nvidia heavily quite recently about how they interact with the open source community, and these sorts of problems are classic examples of what he's talking about.
The choices as an Nvidia user are either use the binary drivers and suffer ugly mode settings on boot, or attempt to use the open source Nouveau drivers which have poor support for many cards and are quite buggy, thanks to a complete lack of documentation from Nvidia.
If you have the option, ATI and Intel video cards have much better driver support under Linux, as both companies offer open source drivers as part of the Xorg and Linux Kernel projects, which support all of the KMS, XRandR, OpenGL and other features you'd expect out of the box.