All UCCs are not ONA (Open and Affirming), IOW, they haven't made their own statements to decide to take this on. I am not sure altogether what this means but you can check this out as usually it will be on the church's website. UCC is congregational and run by its members who can decide what they want to.
Here is the statement of the general synod's statement. I don't think we had any openly trans people in our church when I went there (and I didn't know then). I can't imagine it would have been an issue.
"Open and Affirming (ONA) is the United Church of Christ's (UCC) designation for congregations, campus ministries, and other bodies in the UCC which make a public covenant of welcome into their full life and ministry to persons of all sexual orientations, gender identities, and gender expressions."
http://www.ucc.org/lgbt/ona.htmlThis explains the position as well as giving resources re: welcoming congregations movement (not all UCC).
Personally I think I would feel uncomfortable if I had to explain my gender expression in church.
If you want or need to fight those battles elsewhere, that's another thing.
--Jay Jay