I think this is a windup but just to shed a bit more light on the situation.
I just spent 4:09 cramped in the cockpit of a Cj3 on the way to Chicago. This was not comfortable. The cockpit is teeny and you really have to cram your way in there. The flights your talking about are way over 4 hours.
If you put reserve fuel in the lav area no one will be able to use the lav!
Just have them buy a bigger airplane or fly within the Cj3's limits.