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"?
Ohr Someach gives the halocho as:
Greeting someone with "good morning" and the like is prohibited. One who is greeted should answer softly and, if possible, inform the person of the prohibition.
"Gut Mo'ed" seems as much a greeting as "Good morning and the like" which are prohibited and may undesirably elicit a more light-hearted response since it is unusual which would be another reason not to allow it.
