If it intersected at Wolli Creek (see my fantastic drawing for more detail) then:
Sutherland & East Hills:
a) The Sutherland people could get off at Wolli Creek and wait for an East Hills train to take them around the airport, but instead of going to Green Square and Mascot, the train could divert and head to UNSW. Once UNSW is passed, it would continue through Bondi Junction and into the city.
b) The East Hills/Campbelltown people could get off at Wolli Creek and wait for a Sutherland train to pick them up, and that train would continue through the Eastern Suburbs in reverse, rather than the direction it heads now
c) If Cityrail didn't want to offer services from East Hills/Campbelltown or Sutherland through to UNSW, then everyone on these two lines would have to aligh at Wolli Creek and get a different train service through to the Eastern Suburbs line inc. UNSW.
Everyone else:
a) Get off at Redfern and wait for a train that would go through the Eastern Surburbs line
b) Get off at Central and wait for a train that would go through the Eastern Suburbs line
This plan does have several limitations. The Sutherland line, is by my knowledge the busiest and most used line in the cityrail network. Even if that's not true, then both lines are incredibly busy during peak hourse. This raises the point that peak hour trains would probably not divert through the Eastern Surburbs and all students would have to alight at Wolli Creek for a new service.
If my plan was the plan that they'd take, then Wolli Creek would have to be expanded. Judging from what I see on Google Earth, this wouldn't be much of a problem, as Wolli Creek station sits on dilapidated wetlands and a car park.
The airport line would also have to be brought by the state government, so there would be no bitchy private owner to complain about no commuters through Green Square and Mascot. They were foolish to add the airport line in the first place, absolutely foolish.
As a point of interest, I believe that it could bring the overall level of UNSW up to USyd's level of reputation. From what I've heard, most people prefer Sydney due to it's supposed central location and ease to get to. Personally, I don't regard a 15 minute walk to the university anymore inconvinient than a 15 bus trip, but that's just me. Alas, more people would want to go to UNSW if it's got a reputation of being easier to get to, which would bring it's reputation up.