Most airlines have a temporary ban on any flight deck visits. Its a brave Captain who accepts someone they do not know into the flight deck against the express wishes of the company right now as if anyone found out about it then they could expect to be disciplined for it.
Someone we all know, who works in handling at the airport I am based at asked if he could come on a jumpseat "familiarisation" flight, and he was not allowed to.
If you are a friend or relative of the operating crew you may be allowed a jumpseat but it depends on each airline's current policy.
I would suggest though writing to the airline's Chief Pilot two or three times to see if anything could be arranged that way.
Personally, I reckon if someone has written in, and can be security checked, and knows someone in the airline then it would seem sensible to allow them on. But the guys in the offices don't always see it that way unfortunately.