Thanks for the infos guys!
I will be careful. There is no address of the business in the website which is unusual.
I would not worry too much able "legal" usage of the bladder though as I would plan to use it through Kazakhstan/Mongolia/Siberia and I don't think it would be an issue over there!
Other possibility are the very nippy jerry cans suggested by flyingdoctor.
I could get something built on the bikes to fix these....
I'm just playing with possibilities at the moment. I have to fill my long working days until departure date by something fun to do!