Thanks for the suggestion. If one spells "root" with a 'b', as in "boot" (), then the following sequence indeed sets Media to point to the USB drive next time it is pressed:
DELAY30 MEDIA (0x6f) BLUE (0x1e) RIGHT (0x14) OK (0x13) DELAY5 DOWN (0x15) OK (0x13) DELAY5 EXIT (0x16)
I believe that there is a race condition, as the initial delay is needed (I am not going to play with numbers other than to increase it if I have to, empirically). But I am sorted in the first instance; thanks!