Like Chris Johns mentioned, locking is dependent on lead angle and a coefficient of friction; specifically a coefficient of static friction.
To get a worm gear design to back drive you can increase the lead angle or decrease the friction coefficient, both of which increases the ratio of torque to friction. It will not be efficient, and even though it is spinning it may not transfer much load before locking.
Many "locking" worm drives can back drive when they are already in motion when a force was applied. This is because the dynamic friction coefficient is less than the static friction coefficient. This is why many safety dependent systems also have brakes since the "locking" characteristic of the worm drive is not a guarantee.
If you want to run a worm drive the other direction with reasonable efficiency you may want to look at a ball wormdrive similar to a ball screw design. The ball bearings have rolling resistance which is much lower than sliding friction. Probably too expensive for most projects though.