Certainly I'm not qualified to give out climbing advice, but I keep 150' of 3 mm New England Tech cord (3000lb) in my kit. Just in case.
The problem is finding rappel/belaying devices rated to work with such small cord. I'd be interested what you come up with.
Note this cord is VERY stiff.
http://www.neropes.com/techdata/tech_cord.htm
Oh, I just found
this: