In regard to the article, Portman could appear as a flashback in either the scene where Leia tells Luke about her memories of her mother (indicating that she does not die until after Ep. III), or during Luke's final duel with Vader where he (Vader) looks back and forth from Luke to Palpatine and finally decides to toss the emperor into the core.
A farther out theory I've seen is that Amidala is Mon Mothma (the 50ish woman in white giving the military briefing before the attack on the second Death Star).