How about using the Tacho generators from the brake anti-skid system.
I can't remember what their output is but there are twelve available. One in each axle.
Or there are motors in the ventilation/cooling system fans, galley chillers, toilet system vacuum pumps and a dozen others.
Have a read of By the Rivers of Babylon. A crashed Concorde can be made to move again in fiction. Why not a B777?