zlacker

A horrible way would be to set the target to power on after power loss. Plug the target into a WiFi-controlled smart plug, like the Belkin Wemo. In the event of catastrophic hard lock, power cycle it at the wall.

A better way would be a relay and microcontroller, which you could also potentially use for the USB HID stuff too.

In theory, you could have a HDMI switch, a single Pi and HDMI grabber, and just a microcontroller board for each target machine that could have power button control, keyboard emulation etc built in.