I'll go out on a limb and say that, even were it to be promoted by the estate, a Narnia RPG would have a lot of difficulties--and the biggest of them would be the world of Narnia.
To start out with one, almost alll of the stories are about people who weren't Narnian in the first place. Diggory, Polly, Peter, Edmund, Lucy, Susan, Jill, and Eustace were all from our world. And they were all children. If you wanted to play a powerful warrior like King Rillian or Caspian and his crew from the time of the Voyage of the Dawn Treader, your story would already be outside the standard stories of Narnia. You could play children from our world drawn into Narnia for some specific purpose, but that is already pretty dramatically limiting the kinds of characters people will be playing. And even among people who would enjoy playing from the narrow range of characters, I think a good portion of them might rebel at the specific purpose part of the story.
Second, you have Aslan to deal with. You can't tell a Narnia story without Aslan. He's at the heart of all of them and if you try to leave him out, you've got a setting with lots of talking animals--it's not Narnia. Now, Aslan and what he stands for in the world of Narnia (virtue, etc--we can ignore who else Aslan is supposed to be) are enough to alienate at least a third of RPG gamers right there. (You mean my slutty, thieving, dryad rogue isn't good--this game is for narrow-minded.... or what do you mean I can't play a slave-owning lawful neutral follower of Tash)). However, the game is going to be more limited once you add in his role in the story. In some of the stories Aslan was subtle, in others he wasn't, but in any event, it would take a very skilled DM to pull Aslan's proper place in a story off without having him become the Elminster of bad Forgotten Realms games. And even if the obvious hurdles were passed, there would still be the challenge of getting Aslan right. Playing Aslan properly would be a challenge that I think few DMs would be up to.
So that's it. I think Narnia can be a good source for DMs and PCs to plunder--the wood between the worlds, the garden in the far west, Governer Pug's Lone Isles, every other stop the Dawn Treader made, King Miraz, Charn, the bell and Jadis' riddle/curse, Tash, Tashban, the Tisroc (may he live forever), Rabadash, the castle of the gentle giants, the underworld and its gnomes (ensorcerelled and freed), and Aslan are all excellent sources for players or a DM to take ideas. (And despite what I said about Aslan earlier, I think he's a very good source of inspiration for DMs who want to have the divine impact their world without overwhelming the PCs efforts--there's a lot less risk involved in using Aslan if the setting doesn't depend upon getting him right; even a somewhat screwed up Aslan is better than a lot of alternatives, but if you're trying to run Narnia you have to get him right).
