Ok, let's look at it from another angle. College footballer are what, 22-23 by the time the draft comes and goes? Two years in Real NRL puts them at 24-25. Then they'd probably need some time with reserves to get accustomed to the higher intensity. So they may not even make first grade debut until they're in their late 20s
Then you have the fact that NFL and RL players are completely different kind of athletes. NFL arent endurance, they're all explosive, and only play 30 odd minutes each week, with regular breaks. How are they going to go playing the full 80, in both attack and defense? Are they going to be able to grasp basics of defending in a line in their short time in real NRL?
For the amount of work that needs to be put into it, verses the possible payoff...why would any club even entertain the idea? They wouldn't, as evidenced by the fact that no club has done it yet. If the potential was as great as you think it is, all the big clubs would've been all over it by now