Ok so the main thing I keep hearing about being 100% homegrown is that no matter how great you train your team and players etc. at a certain point there hits a wall that is not overcomeable...
My suggestion is to have enthusiasm slow down less quickly for these teams since they know day in day out who will be on the team... Who will be starting, playing etc. no fear of a foreign player or a transferred ringer coming in taking their spots.
I have a couple of problems with this still.
1. In your initial post, you were not talking about rewarding everyone who keeps a stable team, but only homegrown teams and if the goal is to prevent day trading, then you can't just reward homegrown teams because many teams keep their players and a solid stable roster but have many foreigners on it, thus they are not 100% homegrown.
2. I disagree completely with your psychology courses method. Rewarding teams that are not day trading is like paying a little kid 5 dollars every morning for getting dressed, even though that little kid knows they should just do it anyways. You are rewarding them for doing what they normally do, and it is not as if they are completely some amazing task for their reward, but instead just doing what is expected of them. Rewarding teams for doing what they are suppose to do is silly. The reward is the better fan surveys and better gameshape, resulting in more wins and a better season. There is no need for more rewards.
Murray/Harris/MPJ/Grant/Jokic - 2020 NBA Champs