Psshh, it happened 10-15 years ago and there was never a problem with it!
A great moment in sport was Martin Crowe scoring a 100 against Australia in the opening game of the 1992 world cup, and about 100 people run onto the ground to congratulate him, even though there was still another few balls left in the innings haha.
Actually, when I was a kid, it was never allowed. They would say on the loudspeaker "Please do not come onto the ground after the game" etc etc. But then all the kids would storm the ground anyway, and once over 100 people ran on, the security guards just gave up haha.
I don't think it's a good idea at a grand final or origin, but at a suburban ground it is fine!!!