I'm not sure the NRL could get away with refusing to register him in the long run. I could see a situation where they find a rule to decline him, but as you pointed out, that would very likely end up in court which is a disaster for the NRL who have to fund it themselves, and far less disasterous for Folau, who will easily be able to source crowd funding to fight his case and they won't care if he wins or loses, they're fanatical enough that they'd just be happy to see him fight it. All the while the NRL is watching bad publicity around the game dominate the media, just like the ARU were.he won't be ...
the NRL still refuse to register a contract for him ... which means he is unlikely to find a club that will bother with him (to to mention all the bad publicity and potential rifts with a clubs sponsors).
to have any chance, he'd have to take the NRL to court and see if a court will force the NRL to allow him to play ... which no court will, the best he can hope from that is to get the court to rule the NRL are discriminating against him ... in which case to try to get the general public to pay for his court costs again (despite the fact that he would still have millions left from his court case against RA)
TL:DR Folau is a done playing sport professionally in Australia ... which is why he has take a smallish contract in france (reportedly only about 400k)
The RFL were obviously not prepared to take it on once Catalans forced their hand by attempting to sign and register him. The only difference in the NRL's case at this stage is that no club is willing to take the heat for attempting to register him. If one of them was, I think it would be very interesting to see how the NRL handled it.
I'm sure they'd attempt to save face by declining him, but as I said, that would almost certainly land them in an expensive and lengthy court battle which they themselves may have to settle just to make it go away, exactly like the ARU had to.