This idea is more suited to park football.
To use another sport as an example, it works well in the Shute Shield because well.... it is park football.
When you have a couple thousand people at a suburban park with plenty of grass for the kids to run around, a picket fence for the adults to lean on, reasonably priced beer and a short line up at the BBQ this type of idea works. Even more so when there is about 5 minutes between grades and the clubhouse opens after full time where you can have a beer with the players.
The NRL is a Tier 1 sporting event played in stadia. Sitting on an allotted seat for 7 hours in a concrete jungle whilst paying exorbitant prices for a beer and a pie? No thanks. I have no problem with big stadiums but I personally want to wander down from the pub 20 minutes before the game starts.
As others have alluded to, this would take away from the existing NSW and QLD cups which is where you should be looking at playing multiple grades on a Saturday afternoon.