I believe that you can do that in either aeroplane, in theory. In a Boeing, you always have that privilege. In an Airbus, you can go to 'Direct Law' (I think!). But, also as far as I know, that means that you won't have elevators or ailerons to fly with, just the rudder and the trim controls.......
I think, there is a confusion here concerning "Direct law" and "mechanical backup". When introducing, the FBW-System, Airbus kept a mechanical backup for the stabilizer trim and the rudder pedals - like on conventional aircraft, these are connected with steel wires (The original fly-by-wire ;-)) from the cockpit control to the aircraft tail. You can hence still control rudder and stabilizer trim even after a complete loss of electrical supply or FBW computers. (Whether that is a nice way to control an aircraft, is another question.)
The "direct law" is a control law that the FBW-computers change into, if there is a certain combination of system losses. Direct law, means, as khorton already said, that the sidestick controls directly the aileron/elevator deflection - without any protections or load-factor/roll-rate law. Primarily this happens, when the computer cannot be certain of the correctness of the input data. (I.e. loss of 2 IRs leads to direct law, if the computer cannot automatically identify the second RU loss. (Source: FCOM A320, I assume, A330 is similar.) The crew can get back into alternate law, by resetting the Flight-Control computers after manually identifying the second defective IR.)
So, the easiest way to get into direct law is probably by switching off 2 ADIRUs. May have the "little" side effect of losing speed, altitude and attitude indication, though...