I wrote this pronoun macro about a month ago and Davin (@onidavin) was kind enough to make an exmaple Twee file for it.
Currently only 'she/her', 'he/him', and 'they/them' are included. It would be trivial to add the ability to use custom pronoun sets, but I haven't needed that yet, since I made this to pick pronouns for characters in a game I'm working on.
Here's the syntax:
<<pronoun "she" "subj">>
= she
<<pronoun "they" "posa">>
= their
That's all! tweet at me if you've got questions. @jkap