If I were doing something like this, I would assign new glyphs to each letter (as not to break the image of a fantasy world), and then encode the whole thing with a one-time pad. Doing so can give you all sorts of weird-looking words that have actual value.
Does anyone know how many unique glyphs there are in his book? For that matter, how many are there--including both uppercase and lowercase--in French (I don't know how many accented characters there are)?
And if there are still too many to be such an arrangement, it is possible that each letter was assigned multiple sets (a set being uppercase and lowercase) of glyphs and picking a random number between 0 and twice the total number of letters in French, counting accented letters as unique letters (manually picking the proper case).
I'm not saying that he did this, but it's what I would do.