Same here. I was upgraded to F from J and requested permission from the FA to visit the cockpit while still parked at the gate during a delay at DEL on AA293, cockpit crew welcomed me in and I took several photos and Captain allowed me to sit in his seat and he took photographs of me with my camera! (I am not a celebratory, just to be clear). Afterwards, FA took a few pictures of me at my seat when I requested if she would. There was no problem.

It all depends on the FA and his/her mood, they typically do not like these ultra-long flights and can get moody even with what they perceive to be the slightest inconveniences or irritations from pax, hence they pull out these rules to enforce at their whim.
It's a pity it's like this, it doesn't have to be though. Once UA had slogan which was something like "Come fly the friendly skies". What a bygone era, doesn't exist anymore.