MorganS:
If you can carry the 3h battery pack and 3h is enough time for you then there is no problem.Any gearbox using a worm gear is considered "irreversible" meaning that the output cannot be moved unless the motor is powered. If the steppers don't work out, look for gearboxes with worm gears.
And if you design in a spring loaded split gear to ride on the worm, there will be ZERO backlash in the setup.
Paul