MB Orach Chayim 454:20 says that you are not allowed to say "She'elat Shalom" Mishnah Brurah commentary (41) says that even "Good morning" is prohibited.
Tish'a B'av is called a "mo'ed" in Eicha. Therefore, can I wish someone "Gut Mo'ed" on Tish'a B'av, or would this be considered in the category of "Sh'elat Shalom"?