Interesting concept.
So basically you pick one of your games during the season to be the game where your trainee isnt allowed to play. That could be a different player each week, but always the same game-day.
and then you would pay for those minutes.
Ive always liked the idea of a D-league, this idea is a nice theory - but personally, the 'problem' that you describe - ie: Training or winning - this is what makes the game good. If i have trainee's, and I need to train them, then I need to work out which game during the round to set my trainee's. If im going to TIE one game, then thats where i set trainee one, and then in the scrimmage, trainee two.
The better managers in this game, are excellent at being able to train and win. That is the challange of the game. I dont really want something that takes away this challange. I think its a good challange.
So yer. Nice idea - but i dont like the problem that its trying to solve.