I think you are stuffed.
The bios update would be the best bet.
unless you could fit a tiny kernel with usb support in the mbr to initialise the usb controller and mount the usb device.
if you have some usb keyboard setting in the bios to use at the bios level instead of ps2.
I have had this on an old system, and if usb keyboard is selected you cant use ps2 one (i think), normally you cant use a usb keyboard to change bios settings or get into it.
turning usb keyboard on for bios may enable you to access a usb disk????
go on give it a go!