I'm glad you mentioned gender expression as a seperate entity from gender, orientation, and physical sex. I think gender expression is a fundamental aspect of constructing your social identity (which includes also the three things listed before), and while I think your gender expression explains how you feel ABOUT your gender to others, I do not think it has to define it.
I know many womyn who are female-born, identify their gender as female, and dress boi-ishly or soft butch, etc. I think gender expression is a direct way to relay to others how you feel about society's laws of gender-norms, your personal gender, and how you relate the two. And in no way should society's concepts of how a girl "should look" absolutely determine how you do express your gender. Obviously the pressures of cultural acceptance and laws influence and regulate how you choose to look, but these are more like guidelines which may be broken or bent (respectively) depending on how determined your are to express yourself and how comfortable you are in your appearance.
When you think about it, in fact, culture and society influence our gender expression to an exceptional degree. My gender is fluid, but usually somewhere near androgynous. To express this to others, I usually wear "male" clothing, have built my body up a bit, and have cut my hair shorter than most womyn. However, these are all obviously aspects that I realize society will view and hopefully recognize the way I want them to. That's kind of confusing, so for example: If I go to India and I want people to understand me, I'm going to learn and speak Farsi (or maybe French?), but not Russian, right? The same is true with genderexpression-- you show yourself to the world through body language, verbal communication, and fashion to get across how you want your gender viewed. These things are the language of gender expression. =] I'm writing a bit quickly, so it this doesn't all make sense, let me know.
Anyhoo, I hope that was kind of what you were setting up to discuss. <3