Well, I do not now the specific rules, but attaching something (FLIR, rocket launchers, Movie-Camera Guimbals, GoPros, ...) to the outside of a helicopter will definitely change itīs weight&balance-situation and - even more important - itīs aerodynamics.
This change may not be relevant for the flight characteristics, but we as pilots are not allowed to judge it.
I can imagine, that "aerodynamical changes" need at least a certified workshop or even the manufacturer to evaluate the grade of change and issue an "STC" or something similar.
Remember: This should have to be done for every single mounting point, cause mounting a GoPro to the skid may be totally different from mounting it to the tip of a rotor blade (well, for the rotor to stay in balance, you should mount one GoPro to each tip of each rotor blade ;-) )...
So, mounting any "non certified" external equipment with any "non-certified" method at any "non certified" mounting point, may put at stake the type certification (and insurance!) of this very helicopter.
Anyway: Of course you just can give it a try and hope for the best, but please, donīt complain if this went out to the worst.
Iīd rather be using the GoPro from inside the cockpit...
Thracian