Pretty easy to make a hub device that you can plug the keyboard into and make it transparent to the user. Could even build in a keylogger to capture direct from the keyboard. The attacker would likely need physical access for that, so it wouldn’t be as convenient as the thumb drive in the parking lot attack vector, but unless you’re using PS/2 peripherals (or gluing those USB devices in too somehow), there’s still a fairly open attack vector there, even if you are disabling unused ports in BIOS.