I would not rely upon a Munter Hitch in a 3 or 5 mm line to control the decent of an average adult, especially under the pressure of an impending emergency. There are some micro-belay devices that I believe will work with smaller line, but again it is hard enough to safely rappel with a regular 8 or rack for persons who are not trained and do not practice on a regular bases, I cannot imagine doing so on 3 or 5 mm line.

Pete