I would have it that every club can have one GB marquee player that only counts on the salary cap at a nominal value so you can pay them whatever you want. I would also have a 50% dispensation for a club to sign one player from union and one player from overseas, with some further dispensations for long service players too. With all this you'd probably actually need to lower the salary cap a little as every club would qualify for most of these dispensations.
Paying for it all might not be so difficult as once you get past the salary cap issue, a sponsor might be willing to pick up the marquee player's salary in its entirety. And I also wonder if there could be some form of tax on total salaries in excess of the cap which is then used to fund lower levels of the game or even out the completion.
So Leeds want a player they can't afford from Saints, Leeds Union sign him, then bingo Leeds sign him from their sister/leech club.