I wonder if among the new wave of Talmidei haRambam, there might be anyone using a siddur that uses Rambam's language of prayer exclusively, as recorded in the Mishneh Torah (here).
There is such a siddur, first published in 2006 and arranged by Gil Kobrin/Amminadav, who operates a printing press called Derushah Publications.
It's called Siddur Mesorath Moshe, and was, as the arranger noted, "drawn from the liturgical arrangement found in the legal code of our teacher and master Moses b. Maimon (Maimonides), in the appendix entitled "Arrangement of the Prayer" of the second book of the Mishneh Torah, Sefer Ahavah."
Here is a pdf of that siddur. I haven't been able to find any haskamot or even public awareness of this siddur, besides the Yemenite Jewish forum linked above, but this at least shows there is contemporary evidence of talmidei HaRambam following the Rambam's language of prayer.