As Fly 7 says: "I've flown helicopters with external side baskets with bags secured with elastic cords.". So have I, because the Hughes 269/Schweizer 300 has optional external luggage carriers designed specifically for that purpose. The side baskets attach securely to the aircraft and are certified to do so.
But their design and purpose is specifically to then allow other things (luggage, freight, cameras?) to be subsequently attached to them. All things that Wageslave might consider "hanging extraneous bits off a helicopter".
Clearly there is a weight and balance issue and a safety issue in terms of how luggage is attached to the luggage carrier, but I cannot believe that each piece of luggage (one of which might, for example, be a camera) would need an STC?