Here is another picture of luna, cropped and scaled from a screenshot.
I got the picture by playing this game
http://royalwedding.hubworld.com/dj.html on the hub and playing the rather odd video-creator game without adding any backgrounds.
From the looks of her from current formal sources, she will probably be some kind of background pony with a small possibility of a speaking role with about 5 seconds of screentime, which is rather displeasing since I'd really like to see some more of her personality and interactions with other royalty.
It appears my prediction was fulfilled in spirit partly, luna did get around 20 seconds of screen time with about 17 words spoken in those seconds.
She served her function as a background character well, although I would imagine that she would be more involved in the incident than she actually was in the show.