Insurance is an important limitation to such "innovative hele operations".The main ones that matter are the aircraft insurance and employers' liability for the fire service.
If the underwriters are not told ahead of this "trial" they would simply walk away from any accident, if it occurred. If they were told, do you think for one moment they would be happy to insure the operations? I doubt it very much and there would certainly be a MASSIVE hike in premium. You are legally obliged to tell an insurer about anything that an insurer might want to know about that affects the risk of what they insure.
Questions about needing an AOC are (in my view) not at all certain: it is not at all certain that "gain" could be proven. However,it is undoubtedly the case that an aircraft needs to be insured to fly legally. Just having a cert of insurance does NOT make the aircraft insured, no matter what. Explicit and implied limitations exist which will render insurance invalid.
If H500 does nothing else, can I suggest he encourages JP to approach his insurer, to tell them about what is planned.
As the only helicopter aviation expertise apparently involved, JP's response to the proposition to dangle a fireman under his hele on a rope, does not give much faith in his risk management skills.
Until people have really been involved in emergency operations, it is easy for them to fail to understand that there is a fundamental difference between the heroic actions of a person who gets involved in an ad-hoc rescue by chance AND the person or organisation who plans to get involved in emergency work.