Isao Sasaki's original end song "The Crimson Scarf" (Or "Scarlet" I think) will release on the 25th of April. There will be an event on the 27th to celebrate.
http://yamato2199.net/news.html#minilive
I will assume it will show up again in the epiosde where they leave the Solar System, if not before.
Aso if they are making multiple ending songs, I would guess there would be a new one for each "film" as that would generate CD sales, and make it so that each film as its own ending theme.