I control my monitor from a pi zero hooked up to a cheap vga/usb switch. If your monitor supports ddc/ci, ddcutil makes communicating over hdmi dead simple. The

I control my monitor from a pi zero hooked up to a cheap vga/usb switch. If your monitor supports ddc/ci, ddcutil makes communicating over hdmi dead simple. The button from the usb switch, as well as a wire from the switch's chopped off vga cable is fed into gpio, as is a 4x4 input pad I use to control it from. Pressing D on the pad toggles displays and triggers the USB switch, A50# sets the volume to 50%, B50# brightness to 50%. The big downside is that switching inputs on my monitor takes ~5 seconds where a proper kvm would be instant.