According to my dictionary, the pronunciation of "libretto" (an imported, but already established English word) in IPA is [lɪbr'etoʊ]. We can extrapolate from this.
It would lead to that pronunciation as English likes its schwas and disappearing schwas, just like French does. (Someone else pointed out that "swallowed" schwa is closer to the French pronunciation, too.)
I've lived in parts of the country where "libre" and its counterpart "gratis" were somewhat more common English vocabulary words, and their pronunciations were much more in line with "rhymes with zebra" (LEEB-ruh) and "rhymes with lattice" (GRAH-tuhs). To be fair, those were parts of the country with a bit of a "South midwestern" drawl and some might take those to be lower class, "less educated" pronunciations, but they certainly sounded a lot more like the natural pronunciations of such words in English to me.
