It would actually be best to use a capacitive ladder and you could get about as much voltage as you want.
I have built all sorts of shockers/EMF/ESD discharge units that work off of this theory.
You take 2.4 volts or 1.2 or whatever, pulse feed it into a cap ladder whereby the 2.4 volts or whatever now becomes the ground of the next cap and you literally ladder up the voltage in steps with each rung being another 2.4 or 1.2 volts added..
BUT, better yet is to simply buy a DC/DC converter chip that ramps up 1.2v to 10-12v using a charge pump configuration.
Then, you have it so that as long as the cap is higher voltage than the rail voltage it will allow a one way discharge into the motor.
Better yet, run the DC/DC converter to the entire H-bridge of the motor... After you do the FET mod the H-bridge will handle up to 20V.
That way then the H-bridge consistently is fed 10V from the dc/dc converter.
Problem is that motors don't last too long at high voltages..
Rob