Hey Captain,
I don’t think so. When an electric motor is turned, it acts as a generator, so the only thing that could get harm would be the semiconductors in the driver, but I don’t think the little amount of current would do harm and I assume the driver has some incoming current protection.
For my setup I will use a stepper motor with integrated brake but I don’t plan to update to a Z-20 assembly. I rather will replace the entire Z assembly with something else. The good thing is, for Onefinity Original Series, a version of the open-loop stepper with integrated brake is available (unlike with the Masso steppers)
As I elaborated somewhere in the forum, usually for a machine like a CNC there should be implemented a stop category that considers the risk assessment. And this would be a stop mode that does not let the steppers run freely, but hold their position. In terms of safety, an estop that lets the spindle crash down is a nightmare.