Interesting idea, but it wouldn't come up as first team games, just uncompetative games which is pretty much the same as playing U18 (or whatever the youth league is in Belgium)games.
If they're really that decent, loan listing them should get interest, or you could offer them for loan. Otherwise, you may be forced to push them through into your first team a little quicker than planned a gradually phase out your older or crappier first teamers.
I had the same problem in FM06 with UTA Arad (in 06 Arad didn't have a reserve team or a youth team), and was forced to let a lot of talent leave because I just couldn't accomodate them. Friendlies may help the learn how to play with your senior players, but I can't see them making a huge difference.
That doesn't mean it isn't worth a go.