Allegedly, on or about 04 July 2013, Juan Orti Alcaine sent:
Another good option if everything else fails (and you have configured previously kernel.sysrq=1 with sysctl) is:
Alt+SysRq+ {R, E, I, S, U, B}
Would you expect this to work with USB keyboards? There's more protocols in the way of USB than PS/2, or other schemes, requiring a working operating system.