Mudskipper
I think you've hit the nail on the head about restraint of trade - unless such restraint is permitted because the player waived his rights by signing his contract which had such a provision. (Though perhaps such a contract itself can be said to be in restraint.)
Also, I think he did not ask the NRL for permission so that he did not have to disobey an instruction from them, which may have had some legal wrinkle unfavourable to him should he have proceeded regardless.
As for his competence as a rugby league player to be a defensive coach - Muggleton, the old Parramatta Eel, was quite handy in that role in rugby union; so handy that he changed the coaching profile of professional clubs overseas.
After the defensive success of Australia at the 1999 RWC several English professional teams got league defence coaches. Under Muggleton the Wallabies gave up only one try in the tournament - by the dirt-trackers against the USA.
One thing that Aussie union players don't do as well as league players is to dominate in tackles. If a ball runner is heading north in league the defender wants to get him facing south on his back. You need ascendancy to do that and constant practice.
Whilst a union player would get pinged if he flopped on top of the ball carrier at the end of a tackle and looked to the referee for guidance about how long he could maintain the flop, as in league, the dominating of the tackled player praxis is still valid in our code.
It slows down the attacker's ball legally as he can't present the pill as quickly and as neatly - and if the tackled player is on his back it's difficult for him to protect the ball from the second man in defending player.
Buderus could go OK at the Brumbies if he is able to do the gig.
PS - I recall now that leaguie Chris Anderson got a head coaching gig in the Celtic League in rugby union.
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