I would think it is under consideration. Different markets , different needs.There is also room for jerry cans , which is how people used to do it. Ray Ericksen took a series 2 Landy West of Centre (The name of his book of the trip) in 1967 ; he packed 8 jerry cans as well as the onboard tank.The saying goes , you can't have everything - where would you put it?
I asked that question during the prototype tour and was told its currently 90l for diesel and 20l (vaguely recall) for adblue. Tom said they will try to maximise. Any after market will need to remove the muffler. Wonder if that would be more cost (other than the second tank) coz the rear protection plate and exhaust needs to change. I'm not a fan of boom boxy exhaust systems.