Remember that movie American Outlaws from a few years ago, with Colin Farrell and the kid from Everwood? An old Western set to modern nu-metal? Remember what a big steamer that movie was?
Quentin used a Johnny Cash song in Jackie Brown (and while I love the song and the movie, I thought it was a bit unrealistic to have a hard gun-selling black guy like Ordell listening to country in his car. Just sayin.) and in that vein, I think some plain, hardscrabble country from that era would work well in Inglorious Bastards. If not using actual songs from that period or modern ones that harken that era (ala Steve Earle) then a score that uses some of the same tones with real rustic-sounding acoustic guitars would be tres cool. But using a more old fashioned jazz/big band/blues sound could still work, because not all of that music is so showy and corny and like a lot of ragtime is. (See “The Saddest Music in the World” for further example.)