In Occitan (the Provençal is an Occitan dialect) one of the ways to make diminutive forms is with the sufix [u] (-oo), spelled -o in Occitan and -ou in Frenchisied spellings. Here you have some examples of this kind of diminutives in Occitan:
In Catalan (the "twin language" of the Occitan), for instance, the suffix -ó (pronounced [o] -oh- in many dialects but [u] -oo- in Roussillon dialect, in contact with Occitan language) is used for both genders names, and nicknames as Margaridó (Maggie) or Rosó (Rosie) are usual. I'm sure that in Occitan the situation is the same, but I haven't any evidence (nicknames are strange in names books or in written documentation).
In the case of Liliane, it is not a traditional Occitan name (then it is not odd the "inexistence" of evidences of its use and of its nicknames), but the Occitan form would be Liliana/Liliano (depending of the dialectal spelling) and the nickname could be *Lilio (pronounced [lili'u])>*Lilo (pronounced [li'lu]), spelled à la Française as Lilou.
I don't know if Luc Besson knew the name or if it is just a coincidence.