Good Friday as a greeting?
A person who is in a very good mood might possibly say to someone,
A good Friday morning to you!
Or,
A good Tuesday morning to you!
A simple "Good Friday" would sound odd, however, as you pointed out because of the Christian connotations. As for just "A good Friday to you," or "A good Friday morning to you," well, maybe . . .. Of course the greeting would be particularly apt on Good Friday, two days before Easter.