sorry for not repsonding any time earlier, i was doing some other things...
It isnt as black and white as you describe it and as I think you understand what I said, but I am more than happy to explain.
Every U21 team will start with their best players available to them in their U21 campaign, so would I when I get elected. However I would right from the start select a few players too and add them to the squad that I believe have a chance making it into the U21 team and that are being trained. Just so that every time when a coach looks at the player in their own roster, they get a reminder as to why they are training this player and so that they keep doing this.
I would even try to win games, perhaps we have a good enough set off players and lady luck is on our side and we beat finland in our second game, than why not try and go all the way. However, once it is clear we loose horribly, why not let a few prospects play? They do get experience etc, something as I have said before will have an impact on future games. Why keep playing players when you have no chance in winning and that are after 1 or 2 seasons too old to play? I think it would be better to mix and try and get a balance, so that you can still win a few games and instead off only the coaches that have 20 and 21 year old players also give the feeling to coaches with 18 and 19 year olds that they are involved into the U21 team. if these younger players would play, even if it is on the bench, I would inform their coaches and hope they will watch their players play live...
Your idea to do this once a season in 1 game is great, however I dont think you will keep these players focused to the U21 team with just giving their players 1 game.
I hope it is alot clearer now and if it isnt please point out which part you dont understand and I am more than happy to explain it in more detail...