With both (and only both) RenderAccel set to False and the first Metamodes "1920x1080,NULL;" the X Server will at least properly start (and still crash if the HDMI cable is plugged in/out).
However, it shows a corrupted oversized image (example
) if there's another metamode given, and nvidia-settings will not detect the second display in "X Server Display Configuration", though it does list it as DFP-1 under GPU 0. Also, that monitor will remain in standby.
RenderAccel may remain true, apparently.
The monitor is listed properly in Display Configuration, I just had to actually select it in "Model:". The crash then occurs once it's enabled, regardless of the chosen resolution (eg Auto).