I don't know. The 75% is only for season tickets to and from home to work, but Collingwood is a bit unique. No one ever tried it as there was little need to. They did muse on buying 16-25 railcards for everyone, I don't know if it's still the case. You can do as you wish on weekends (as long as you're over 18, as curfew otherwise applies).
It's a personal thing. I liked it. Free food, cheap beer and all living expenses paid for. Take a laptop if you have one, can get dull on quiet weekends. Money's not great but it's all in your pocket. If you've got decent room mates, it's a blast. You'll make good friends down there, make the most of it. The Navy are decent too, and you'll get free reign of their facilities, gym, pool, sports, bar, everything.