Hello, I’m not quite sure in the rules in different countries but I believe in the US you have to have special permission to go on the tarmac. This could be something that the captain is able to issue but I just couldn’t imagine that would be the case. I would look into your local rules for those things to find more information. I agree that would be a very cool experience and I hope you are able to do it :)
You could surely give it a go! Do know that safety regulations at most major airports became way more strict after 9/11, so I don’t estimate your chances to be that great.
We have a saying in Dutch: nooit geschoten is altijd mis, roughly translating to: you will always miss when you don’t even shoot.