I have begun to work closely with a diverse group of members of the Jewish community in my city for a non-profit organization that deals with issues that cut across all levels of Jewish observance. As part of this, I am in close contact with Jews that lead non-Orthodox congregations that have not received smicha from a recognized Orthodox program. Is is appropriate/mandatory/totally/"nice"/ inappropriate for me, as a recognizable "Orthodox" Jew, to refer to these leaders with the title of "Rabbi"?

When in private meetings why not go on a first name basis? In public, refer to them as "my colleague" or "Dr. so and so, rabbi of Temple Beth Shalom" or something like that. HaRav Feinstein in teshuvot generally refers to them as "rabbi" while referring to Orthodox rabbis as "HaRav".