The plausible "system" would be a points system. The reason it benefits clubs is it puts downward pressure on player payments. Clubs with deep pockets can only bring in a limited number of players and hence they cant keep forcing payments up by bringing in middle of the road players at an over priced amount.
The real issue would be how many points for a player from another SAAFL club. It might stop Div 1 and two clubs poaching players from neighbouring lower grade clubs.
The "import rule" could not work because in country leagues an "import" is pretty easily determined. e.g doesnt live in the town or surrounding areas. If you considered the clubs surrounding say, Gepps Cross where would the administrators start being able to say this player is an import?