Desktop PSU registering signal from 220ac input
Do "current+old" generation desktop pc's have any built-in means of registering a signal that is modulated on a (220v) AC power output?


No. They actually have input filtering in order to meet regulatory requirements -  intended to prevent noise from leaking out to mains, though.


  if power supply units (PSU)'s have an input voltage sensor which is somehow fed into the motherboard somewhere


The outputs of an ATX PSU must be isolated from mains, which would add a significant cost to such sensor. Standardized desktop and laptop power units are heavily cost optimized.