Raspberry Pi definitely works! I have a project you can take a look at; you'll have to modify it slightly since you want a keyboard rather than a joystick, but

Raspberry Pi definitely works! I have a project you can take a look at; you'll have to modify it slightly since you want a keyboard rather than a joystick, but they're both HID so the majority of it should work pretty much out of the box: https://github.com/saulrh/composite-joystick https://github.com/saulrh/composite-joystick.