I think it'd be nice (I myself was thinking of Digory before I saw your post, AlwaysHis), but I think it'd have to be very sparingly used. Like how on LotR, Galadriel narrates the intro of FotR, but not the end. She sort of narrates in the middle of TTT, and I don't remember her narrating in RotK.
But there are some parts of Narnia that are written very much like a narrator. For instance, I love the way he writes stuff to the reader like, "Have you ever done this?" and stuff (not that I want to see that in the film, just good for the book), but definitely the first line about Eustace in VoDT would be good spoken out, but by someone in the stories, preferably a man ('cause it feels more like CS Lewis is telling us the story).
hm, in TMN, the first two paragraphs could be used.
LWW could start out "Once there were four children whose names were Peter, Susan, Edmond, and Lucy. This story is about something that happened to them when they were sent away from London during the war because of the air-raids. They were sent to the house of an old Professor who lived in the heart of the country, ten miles from the nearest railway station and two miles from the nearest post office. He had no wife and he lived in a very large house with a housekeeper called Mrs. Macready."
In HHB, I could see some alteration, maybe something like "In the Golden Age of Narnia, Peter was High King in Narnia, and his brother and his two sisters were King and Queens under him. In those days, far south in Calormen on a little creek of the sea, there lived a poor fisherman called Arsheesh, and with him there lived a boy who called him Father, though whether he was his father is another matter. The boy's name was Shasta."
I can't think of how PC could be narratated unless they start from Prince Caspian's view before he blows the horn, calling the Pievensies. It could be narrated by Dr. Cornelius.
In VoDT, not really sure, but I would definitely like to hear the first sentence about his name.
I don't know if there'd be a good way to start TSC with narration. I think that'd look nice if it just goes straight into Jill crying behind the gym.
TLB could either go right into it or could start with the first several sentences describing Shift and Puzzle.
Anyway, I know I got a little carried away, but that's how I think the narration could be done for each of the stories.