Countries could do this as part of prep of juniors for the future without hindering their chances of the current senior competition.
I remember, a couple of years ago, I had the chance to <ahem> 'acquire' a recording of that year's ANZAC test. It also came with the Junior Kangaroos v Junior Kiwis game that was played before the main event. Most interesting and a very good match to boot. It also gave a chance to see tomorrow's star players in action.
I think it would be an idea worth trying, and would help turn each round into more of an all day event instead of just the one game, with people turning up close to kick-off and disappearing as soon as the hooter sounded. Obviously, many spectators would still just go for the main match, but I think you'd get plenty who would be tempted by another earlier game. Maybe have live bands playing too before and between games, generating a sort of 'festival' atmosphere.