I might just point out that the same "Splash" lubrication will occur regardless as to wether the Gearbox is being driven by the Engine or the rotation of the Propshaft. The exception being that the design of an Auto box, and the lack of splash to the torque converter, being what primarily precludes it being towed. I regularly "flat tow" ("A" frame) cars, as do others, behind my motorhome and have done many hundreds of miles accross Europe too. My Fiat Punto, went to Spain in this manner 3 years on the trot, and was sold for further use when the P-107 (semi Auto) came up. Smart cars are a favourite for this (pre 2008Models).
Throughout North America, thousands of vehicles too are actually "flat towed" (all 4 wheels down) and do many thousands of miles. As I did with my Toyota Tacoma 4WD Truck (20Kmiles). This had well over 150.000 miles on it when my Son sold it, for near as much as I paid for it, and was still going strong. So in my personal experience the threat of damage to the gearbox is something of an urban myth.
Pete