The Chooks don't have to use paper bags, but they can and do use third party deals to get the big names. This is the NRL's loophole to allow the few rich clubs to keep dominating, despite the salary cap giving the façade of a level playing field.
The problem is that the 3rd parties are supposed to be at arm's length from the club - the agreements may not be negotiated by the club as an incentive for a player to sign a contract, nor can they be guaranteed by the club.
However in almost every case, the 3rd party deals available will indeed be part of a club's incentive for a player to sign a contract, and hence in breach of the salary cap rules. Which are not enforced and in their current form probably not enforceable.
