However arent most power limits, with a de-rated engine related to transmition limits?
Not necessarily. In many twins, you can reach engine limits well before transmission limits. It depends on the make and model, on the altitude and temperature, and on other things. On the model I fly, on a cool day the first limit I reach is Vne, but as it get hot I may very well reach N1 limits first, at least max continuous limits. I may reach transmission limits on takeoff, but not likely in cruise.
The 5 minute limit is for takeoff, and maybe to stop your too-hot approach, but not for photo work or anything else. If you're flying so slowly doing photo or any other work that you need to pull into the 5 minute limits, you're very wrong, and endangering both the idiot photographer who is asking for this, and the idiot pilot who agrees to do it.
If one engine quits at a bad time (and don't they always quit then?) then the other one had better watch out, because I'll pull all the power I need from it to keep flying - temp, N1, and everything else be damned. But that's not even close to the same thing as pulling all the power it can produce just because I want to go a little faster. My company bills by flight time, and the faster I go, the less money it makes, thus the less it wants to pay me. The driver referenced in the original post is plainly a pure idiot.