And as someone said earlier in the thread - how would QF know if WP's using the lounge were not flying Qantas? There is zero method to track this domestically.
Domestically being the key word there. As I said in the post above maybe the core issue are international lounges, especially because Qantas doesn't own the international terminals so may not be able to expand the size as 'easily' as they can do in terminals they own.
Having said that though domestically I have noticed, in Canberra in particular that the people behind the desk often use a check sheet when people arrive. Maybe this is what they use to keep track of these things.