If you're spelling it Luellen, and pronouncing it clearly "lou-ellen," then I think it can work fine. Llewellyn is pronounced somewhat run together - there's not a clear break between "lou" and "ellen," such that it can almost be "lle-wellyn," if that makes sense. It's not exactly that, but anyway.

I like the Lulu nickname for when she's small, and that she can trim it to Ellen, which is a perfectly lovely and classic name, when she's an adult, if she chooses.