I supposed you can do so. A friend of mine has once gone thru security and immigration in Pier B so to see her parents off to HKG on a CX flight before she walked over to Pier C to join her SQ flight to SIN.

In terms of the line at the immigration, I think SYD has a lot of things to learn from HKG. At least the immigration should do something to move some of the line over to other pier when one pier is very crowd. I once lined up for an hour at the immigration to get on a flight of 3 hrs to AKL.