Helen was the first choice in the rumor mill when they announced the show was going to be going to Sydney. Keep in mind that they announced the Sydney production not so long after the London one opened up.
At that point, Helen was doing the show in London and the fans figured that she would go because she is from down under.
There have been interviews that bring up this rumor. She says that she would like to do it considering the fact that it is a homecomming of sorts for her. But, nothing has been talked about between her and the producers yet.
I don't think that Idina would do the show yet again. Although, she is releasing a new album and is trying to have it reach a more broader audicence then the broadway fanbase she has gotten. She is trying to go into the mainstream pop area of music.
That being said. She could do the show in Sydney and consdier it a business move. While she is there she can promote her new CD and get a bit of buzz going about it internationally.
"If you try to shag my husband while I am still alive, I will shove the art of motorcycle maintenance up your rancid little Cu**. That's a good dear"
Tom Stoppard's Rock N Roll