I don't think they'll weigh a ton when wet if they are made from the same material as sitting bean bags. As to cost, that depends what companies want to charge (same as sitting bags). Spoke with Marc Fisher last week and he said kneeling rolls are very easy to make. I reckon a kneeling roll made of bean bag material will outlast a pilates roll used outside.
I'm not trying to start any debate about which rolls can be used. It just seems a little like re-inventing the wheel. I assume that other shooting disciples already have rules about the bags and the kneeling position so why not use that information and save effort from volunteers.