Do you see how low the back rest is? I think you would need to modify the futon frame in the same manner, turning the double into a super single like the example you just posted. I would be inclined to do some welding after chopping the futon frame up, and make brackets in the wall to support the backs of the armrests. Reason I am talking about this is I have that futon, and I have to pull it away from the wall in order to fold it down. Any chopping though, and you are looking at getting a mattress or cushions to fit.
Also, some juristictions would not consider the seating legal for passengers. But if you aren't using it as a passenger hauler, but more for camping, I'd say it would rock! Checking out that web link you posted... pretty cool! I like to design my own layout, but I suck at cabinet making! Nice that there is an option from a vendor that appreciates vans!