A Ram Air Turbine (RAT) serves as an emergency source of electrical and hydraulic power. It has no operating time limits, airspeeds, or altitudes.
According to this link (Question 6):
https://cramberry.net/sets/63118-757-hydraulics the RAT on a 757/767 requires a minimum airspeed of 130 knots. I don't know what the limit is for the 787, but it is clear that this equipment is designed and certified to provide emergency power in the air, not for electrical braking on a landing roll.
To claim that it has no operating airspeed limits would logically imply that that it could produce sufficient power if it was deployed while the aircraft was being towed. That would solve the battery rundown while being towed problem - just deploy the RAT and the aircraft will power itself!
