Interesting discussion. Fun on a very cold February. I have read all posts on the topic with interest. Writing is a weakness for me so please be gentle.

First of all, can gender be a verb? I think so. I had to laugh at Jill's example. This person was gendered correctly (or see the similar verb described below).
http://susans.org/wiki/MisgenderI searched for the prefix Trans- which means across, beyond, through
I was able to apply the trans- prefix to the below example.
1. I will form a sand castle.
2. I will transform the sand castle to a sand palace.
3. The palace was transformed.
By adding past tense, you can turn a verb into an adjective such as hammered steel or painted fence. This would not work for the adjective gay because it does not appear it can be applied as a verb, so the term "gayed" could not be applied.
In regard to transgendered, I guess it can be used if: a) gender is considered a verb, b) it can be applied past tense, c) trans- is an appropriate prefix for "gender" used as a verb.
I know. Not very helpful (or even correct) but was a lot of fun.