Well, I don't think that would be very possible if you're talking about white Australian, because they never had a king or queen. However, if you mean an aboriginal princess, then sure, why not?
Hmmm, could be ok, although as others have said- she'd either have to marry a prince like Tiana or be an Aborigini (sp?). Besides, they have been trying WAAAAAAAY too hard to make the newer princesses "feminist" and "empowering"... so basically they ruin them and make them less feminine and romantic than they ought to be! You know, make her act like she doesn't "need" a man, be snippy and sarcastic, etc. ;(