Do hot air balloon pilots file flight plans?
As a balloon pilot: no we do not.  We do fly regularly in classes G, D, and E and will occasionally wander into class C; but it is very frowned upon...and you better have an aircraft radio or at least call the tower to let them know you are there.   Of course balloons always have right-of-way so if you see us get out of the way; we can't control where we're going.