It is very very possible.
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:http://perso.orange.fr/filsdebelesta09/contenu/pageoccitane.htm
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 Margarid
) or Ros
) 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.