flying for a mark is an action that creates incidental contact. You're not so much holding a player out by instigating contact, but maintaining your balance in the act of leaping on a player or pack of players. Even then, hands in the back or on the shoulders is still not allowed.
The rules of permitted and prohibited contact are saying you can fly for a mark and make incidental contact, but you can't deliberately (and potentially dangerously) make 'hold out' contact in the back or with a part of your body not listed as permitted (for example, it spells out that you can push in the chest or in the side with open hands if the ball is withing 5m, but can't push in the back). Kicking people is clearly prohibited and there's an argument that lashing your foot out (which is what Greene does) can be deemed a kick.
In my strong opinion, the current laws already allow for an interpretation that would deny Greene doing what he did/does. However if it makes people more comfortable for it to be spelled out further then I'm ok with that too. But be prepared that if the rule says 'no studs out' that would mean flying for a mark too, which is probably why it isn't already in. The rules in their current form allow for the spirit of the game, which is - high flying marks are good (but no hands in the back) and sh!tful acts like Greene's can quite easily be ruled against with interpretations from a number of clauses (ie, you can instigate front and side contact but not with your feet, you can't kick, you can't be unduly rough, you can't perform a deliberate action that has the potential to injure - take your pick from one, some or all of these).
