Wash an electric motor? Wow.
Wrt aquarium charcoal, you mean pack it into the gap between the armature and the windings and everything? Sounds like a momma bear to clean the motor afterwards. Might be easier to wash it as you suggest and let is air dry in our nice hot dry desert for a week or two while I wait on a hose and brush rolls, etc..
So just submerge it in a mixture of Big Orange and hot water, agitate it, then rinse it out thoroughly and air dry? Do I try to remove the brushes first of just leave them in for the treatment? Labor wise that is the easy way to do it, just makes my boys retract thinking about it. Tell you what, that vac gets turne on outside the first couple of times, with a 10 BC fire extinguisher handy.