I know I'm a little late to this thread (been away). This is something I've already done. Web and WoL control of a PC's power and reset via the motherboard headers in parallel with the front panel buttons.
A Pi Zero with usb wifi, powered from 5v standby, 3.3v from the PSU for state monitoring (only present when PC is on) along with connections to ground and the motherboard headers.
Other model Pi should work as long as they don't draw more current than the PSU can supply on the 5v stand by line.
Details (and software) here: https://github.com/thagrol/fakewake
Attempts to contact me outside of these forums will be ignored unless signed in triplicate, sent in, sent back, queried, lost, found, subjected to public enquiry, lost again, and finally buried in soft peat for three months and recycled as firelighters