The 'rule' regarding players not based in Wales has never been clear cut. Gatland has stated on numerous occasions that he will simply choose the Welsh based player if the decision is a 50/50 one. This makes complete sense, as the likelyhood is that the non Welsh based player will miss a certain amount of training sessions and AI games. It's also harder for the Welsh management to keep a close eye on them by going to the matches.
Hook will surely have made sure that his contract is iron clad in regard to all future Welsh games and training sessions. I'm sure he will continue to be involved with Wales during his time with Perpignan.
I'm excited to see how he grows as a player out there. I'm fully expecting him to return a much better player in the same way Stephen Jones did. Good luck James, someone of your talent deserves to play in the position you feel at home in, i.e. 10. Ospreys to see what they have let slip through their fingers!