The way I've seen this handled in other forums is another entry below the avatar like "Pronoun: she", like the location tag. If you don't set it, it doesn't appear at all.So how do you propose this be displayed? It seems a bit awkward to have a note under our username that says something like "Please refer to me as 'she' (or 'he' as the case may be)."
Do you think that would be awkward? As soon as it becomes widespread everyone should understand the meaning. It's kind of similar to setting a foreign country as your location to indicate your English skills aren't perfect, so people who don't know you can immediately take this into account when talking to you.