According to the Google dictionary, transgendered is not a real word and it defaults to transgender.
I would like to believe either are acceptable to use, at least it sounds better in my mind to say transgendered instead of transgender but I can see the argument for both.
If you want to create a poll for the community and let them decide which word they prefer for the category, then we'll have a case to take to those in charge and see that it happens. Just send me the link when you do so I can keep an eye on it.
(I'd give the poll a two week deadline.)