Just about to leave for the weekend, but I checked in to see if there have been any further developments - I have to admit the back to back 4051 idea is pretty good, although I would use the INH lines as well (so four Arduino port pins per 4051) to make them disappear from the scan lines when they were not actually generating a key press.There's still the USB power problem to untangle, but I admit it's a good compromise.