Why don't you use disper ( http://willem.engen.nl/projects/disper/
)? Launch it when the acpi event (Fn+F4) occurs. That would be a scriptable solution.
Monitor hotplugging is very much allowed, as long as you have DynamicTwinview active (by default it is active). If you use the GUI nvidia-settings, then also you should be able to dynamically (de-)activate any attached monitor.
Oh yes, in my usage, till date I haven't missed anything from xrandr-1.2 while using nvidia-drivers.