In his excellent book, Anatolica, Richard Clogg mentions the presence of “Armenian-speaking ‘Greeks,’ who employed the Greek alphabet to write Armenian” in the 19th century Ottoman Empire. That’s cool. Before the invention of the Armenian alphabet by Mesrop Mashtots in the early 5th century, some Armenians wrote in Greek too, but that’s because there was no existing Armenian script at the time.
P.S. Clogg references Manouil Gedeon, “To kirygma to Theiiou Logou en ti ekklisia ton kato khronon.” Ekklisiastiki Alitheia, 7 (1888), p.200. When I plug the title into Google it returns this. If anyone can make out the relevant bit, please comment below. It’d be good to know what the nature of the reference is.
Edit (2/18/19): Thanks to Twitter user @kilikyaci for pointing out that there was a well-known community of Greco-Armenians in the Armenian empire known as Hayhurum (or, Hye-oo-Rum, which translates to Armenian-and-Byzantine), who spoke Armenian and adhered to the Greek Orthodox Church. You can read more about them here.