Yes, that kind of sums it up.
I've flown helicopters with external side baskets with bags secured with elastic cords.
The Go-pro is tiny. There really is no issue with weight, aerodynamics or security in the proposed location.
The concern is the legality, and I'm assuming it's something that requires 'approval'. But, before asking the CAA, I just wondered if anyone has had any previous experience - fixed wing or helicopter?