I kind of did it. I don't know if this works for you as well but, I just removed the SATA cable for the hard drive that contains the OS, booted the pc then waited for a few seconds till the keyboard was already usable (I used numlock or capslock as indicator).
I don't know but maybe this explains it.
Usb devices wont load until you boot to the OS. So if you will remove the OS (hard drive that stores the OS) , my theory is that it will force it to load the USB devices at post to "seek" for recovery options. Because it finds an available OS to boot at and if it doesn't for an amount of time, it will give you access to your USB keyboard (if you disabled it at bios) for you to recover and "help" it find an OS.