Just back of the envelope of course, but I should imagine there'd be all manner of nasty aerodynamic effects upon the hose / drogue for helicopters of whatever type, not to mention that pesky tail rotor on the lead ship.
Helicopters in formation typically fly with step-up as opposed to step-down as do the planks for just that reason (also it's easier to look downward in most helicopters than in most planks due to the positioning of the glazing). I see little advantage over using a C-130 or other turboprop where there's all sorts of room for tanks & other machinery and plumbing. Most tankers have a lot more range than what they are fueling (I except carrier-based tankers) and far greater fuel capacity.
Rotor downwash on the hose and on one's own rotor while bringing the hose under one's own rotor... I shan't sleep tonight for thinking about it.