Here is a video where I demonstrate how to connect up a Raspberry Pi to an Arduino via USB serial, so that the keyboard characters from a local machine, can be output to the remote machine even when it is in the BIOS. This video show a setup that uses the Pi camera, which outputs a hardware accelerated stream, so there is not much CPU usage. I have also managed to get a USB capture card to work and will be showing that in a later video. You can check out the code at the below link:

https://github.com/macpczone/keyboard-passthrough/tree/master