Currently nvidia-xconfig does not support the HardDPMS option.
Since the default value was changed since the 440.x driver it might be a good idea to add this to allow for an easy change.
This is required for VirtualGL to work properly for example, see: https://virtualgl.org/Documentation/HeadlessNV
Currently we modify the xorg.conf to add this, but an option for nvidia-xconfig would be preferred to accomplish this.
I hope someone can forward this request to the devs :)
Initially created a ticket but was told to post in the Forums instead.