The just a simple thinking.
Power On/OFF, Built a Jump socket connection to ATX power supply and MB/PowerKey;
Built a MCU with IR receiver, using Standby Power for ATX/Monitor +5V(check PC On/Off state)
MCU using USB to RS232 connection to PC
MCU monitor IR receiver and process command.
Install a service/deamon to PC, and setup USB to RS232
receive USB to RS232 packet and process to emulation Mouse/Keyboard and Others input event
Standby power and USB to RS232
Signal of Power On Signal of PowerOn
ATX Power supply <————————–>[MCU + IR Receiver + USB2RS232]<—————————-> MainBoard
so hardware is very simple.
5V MCU like Microchip PIC/Paduake FPPA
IR receiver http://www.vishay.com/ir-receiver-modules/
USB to Serial bridge (FTDI FT232BM/Prolific. PL-2303/Silicon Lab. CP210x)
on Linux has LIRC(Linux Infrared Remote Control Project)
on Windows has WLIRC(it has porting LIRC)