If the top ups come from the NRL themselves, they should copy how rugby unions sidestep salary caps by putting most of it in retirement funds. You then pay out the fund ten years after they retire from rugby league, with a non-compete clause in place for that ten year period. That way players who do come over have to stay in rugby league and not switch back. Players that do switch back would sacrifice a part of their fund for breaching their contract.
Yeah my main concern is that players will use it to bump up their price in leaving and coming back salary cap exempt. Clubs would end up nearly encouraging them to leave so they can get them back salary cap exempt.
If you started it today, you'd have to make it a 1 way street for exemption from today. Meaning players currently in the NRL would never be eligible for a return under a salary cap exemption. But then we have an issue of highly talented youngsters making certain they come through Super Rugby or something so they can go to the Roosters salary cap exempt..
It's riddled with issues hence the reason it hasn't been done yet and why they've been looking at it for so long.
It'll be interesting to see what they come up with.
My thoughts would be, we just have a 1 year window where we open the door and completely pillage the rival competition of their talent.