This is incorrect. The players agreed that 100k from each teams salary cap would go towards funding the RLPA. The NRL has no legal leg to stand on if they withheld that. The NRL are merely paying on behalf of the players.
The remainder of the funding is from the NRL purchasing image rights for the players. Sure they can break that contract, but good luck promoting the game without the consent of the players to use their image.
Sorry mate, but industrial relations is one area I have a lot of practical experience in. The NRL could legally do as I alluded. I know as it happened in Victoria (iirc) when the Victorian Liberal Government unilaterally ceased all union fee payroll deduction payments for its public sector unions. Thus all victorian unions had to spend months getting members to set up direct payments arrangements with them as otherwise they were all but insolvent. They had no legal leg to stand on.
As there was a fear this would spread, many unions in other states and at Federal level followed suit moving away from payroll deduction methods where it was paid thru the employer deducting the fees.
Secondly, the agreement that permits this is the CBA. Come 1 November that agreement has expired and is no longer enforceable.
Further the problem with the RLPA's funding model is if you had a player or two objected to it on grounds it is defacto compulsory unionism, it could be legally challenged. Rightly or wrongly, that is the law.
If the RLPA had to get direct fees from its members, they'd be selling the fact that the fees are tax deductible. I know that union fees are tax deductible as even the ATO has said that for decades.