My belief on these kinds of days, though I never had them: Go as a guy. Seriously. If it's opposites, that means your default is female. If you go in a dress, skirt, etc. that day, that's just the standard guy response to such an idea, since his opposite is female. It's an odd circumstance, but that's the one time anyone who has transitioned to, or plans to transition to female, should go as a guy.
If you get asked, you can say that you're not up for that kind of idea, or, you can say that you're doing what every other girl is.