I read about the case of Dutch too. It was given as a reason that there are so few Dutch creoles. Sign languages are also hard to get access to and I've been told, that deaf people will run berserk, if somebody who hears thinks up a new sign. Gypsy languages also cannot be learnt so easily, but I once saw a course anouncement for Roma. Palawa Kani, a Tasmanian conlang, is kind of forbidden for everybody who isn't a Palawa.