My B2910 tires, front foam filled, and rear liquid filled, have been that way for 19 years now with no ill affects. R4 tires. Do not think turfs would be any different, although better in as much as they are larger and will hold more liquid, which means they will be heavier.
The liquid within the tire should have minimal affect on the transmission, since the only connection to the tire is the friction between the tire and the liquid. So the tire will start to spin before the liquid does, so the liquid does not have the same affect on the transmission as solid wheel weights would.
Beware of misinformation that is out there. And remember that sometimes Kubota includes recommendations that are unreasonable in their manuals. For example, someone reported that their operator's manual said to remove liquid ballast from the rear tires before installing a backhoe. Nobody in their right mind would do this every time they put their backhoe on...
My guess is that liquid in the rear tires would affect the transmission the same way an implement hanging on the back of the tractor would. And tractors are designed to have heavy implements on the back, and to actually use them in contact with the ground, which adds more stress to the drive train, stress it is designed to deal with.
Just my 2 cents...
