I think the NRL would take a very dim view of this, and rightly so. WT would love to be able to toss a marquee player like Marshall an extra million to sweeten the deal. Club funds and salary cap restrictions don't allow them to do this, so it would be a nice little workaround to loan him to an overseas Rugby club in the off season and have them pick up the extra. Rugby Union halves can generally stay out of trouble and aren't expected to do any work in decence, so Benji would make some very easy money with minimal injury risk.
If it were allowed to go ahead it would be a massive salary cap rort, hell the league doesn't even want Danny Buderas doing off season work as an assistant coach in Union.
Rugby 10's are the busiest players on the field. In attack they operate under much greater defensive pressure because of the proximity of defenders , positioned significantly closer in RU than RL. Even a second receiver in RL still has plenty of metres between himself and the defence and therefore more time for decision making. In Rugby the offside lines are governed by the last feet of the defending side at the breakdown. Tell Jonny Wilkinson or Dan Carter they're not expected to defend. The fly half is targeted by most attacking teams and generally has plenty of traffic coming at him, in particular loose forwards
gone are the good old days where the five eight got a free pass in defence. I cant remember which forward it is, I think it is SImon Poideven, tells the story in his after dinner routine, where the coach is going off at Mark Ella missing 3 tackles, and he is thinking about time some one had a go at the protected species, than he turned to the forward and blasts him, how come you let 3 blokes get to Mark….
Those days are well gone, they run everyone at the 10 these days.