It would be very hard for either of them to come back to RL.
They went off to AFL for the riches, dissing the NRL, only to fail and come crawling back? By which time they have been out of the game for a few years, and changed their body types?
If they fail at AFL, they will chase Rugby Union dollars, probably in Europe. Especially Hunt, he will never play in the NRL again, nor should the money grubbing Karmichael. Lets face it, he was at best an Origin winger/back-up fullback, since replaced easily by Boyd. There is a lot of young tallent comming through and there is no place for him at rep level in the game now. He would just be another run of the mill fullback.
His defection hurt because he had been groomed as Lockyer's successor at the Broncos, which he turned his back on. He will play out his career in obscurity in France, which is more than he deserves. He also talked a lot about how he didn't like Rugby League. Good ridence.
I think Izzy can still maybe come back to the NRL simply because there is still a year to go before he starts at AFL level, and because he appears to be useless at AFL; but the door is closing for him. Izzy is in a different situation, as he clearly chased the money, but has been very uncomfortable with his new job. Hunt wanted to leave and play AFL, where as Folau was chasing the money and would rather play RL.