Who uses/used the Rambam's Hagadah for the Seder (besides Teimanim)?
Modern Rambamist's also use it. These are non-Yeminite communities who have decided, usually as individuals, to follow Mishneh Torah almost exclusively, instead of later authorities like the Shulchan Aruch.
I know of one such community in Beit Shemesh here in Israel, Ohel Moshe.
I intend to, if that's of any value. I also hope to put together a Heb/Eng version for next year.