Originally Posted by marcuslai

Presumably nothing in the rule books against it. This happened less than a month ago on CX. Upon boarding, a friend of mine mentioned to the ISM he was very interested in planes and would appreciate a cockpit visit after his flight HKG-TPE. The ISM came back a short while later asking him to go to the cockpit now as the captain thought it would be more convenient to have the visit now than after landing at TPE. While chatting inside the cockpit, the captain invited him to sit in the jump seat for take-off after teaching how how to use the oxygen masks etc.. He stayed in the cockpit for much of the flight, wearing a headset and listening to the cockpit chatter. It was quite a dream come true for him.

On the return, he mentioned the same to that flight's ISM but didn't get a cockpit visit.

I think it really just depends on your flight crew.