It sounds like you are essentially trying to determine how much torque it will take to move your servo while it is not being powered. If this is the case, you would probably need to measure it, since it is not a typical specification to give. You might find the "Force and torque" post on our blog helpful for understanding how you might go about that.
By the way, please note that it is generally not great to constantly back drive a servo as it can be rough on the gearbox mechanically. However, it sounds like you are only going to be doing it as a fail safe, so it might be fine for your application.