Go for it. Boring.. Yes/no depends on what tunes are in your head.
Legally you just have to turn off at Werribee and come back through Laverton, Williamstow, Footscray or come back onto the highway via Grieve Parade for a faster ride into Footscray or Flemington.
Audax map here for their ride directions
Where you have to turn off the freeway you can use the Bikely map from there.
Oh! And if the road is busy it can be somewhat noisy.
Does riding East Link (with 30,000+ other cyclists) before it opened count?
I've ridden on the freeway between Byron Bay & Brunswick Heads in Northern NSW many times. Wide shoulder, clearly marked spots to cross the freeway's entry & exits ..... and trucks thundering past at 110kph.
OK, they might go that fast on the back roads, but there aren't nearly as many of them.