You can't exempt all Academy players. You could give a discount against the cap though (e.g. max of their salary or 50k/player). You also shouldn't count the likes of Hardaker under any system. If not developed by the club as a junior they shouldn't count at all.
Even if you were to introduce this you'd also need to start from scratch - no giving the likes of Leeds and Wigan an immediate free kick.
I'd also exclude exemptions until players progress - e.g. 50 games. You don't want clubs stockpiling kids just because they developed them if they're not going to get a game, and this should be about quality not just quantity.
I think you'd also need to introduce catchments. Its not fair to the likes of Cas/Wakey if Leeds cherry-pick kids from their area and count them as their own (or Warrington from everywhere). How would you treat clubs with virtually no natural catchment (e.g. Salford)?
We all need to be developing more players, but while there are lots of ideas the devil's in the detail.
One other idea I like is the father-son rule they have in AFL. If a kid's dad played more than 100 games for a club, they get first pick in the draft. We don't have a draft so I'd just have a cap deduction. It would hurt us today, but I like the idea - recognises history which is something our sport is not good at.