don’t ask why you would, but I just realized that when your iPod is connected in manual mode you can hit play and the music there plays through…
Select your iPod from the Source list… you should see the tracks on it. pick one and hit play. This just happened to be by accident actually but after trying it on a few tracks, it really does seem to work. I checked out the same songs in my main library file and they were not playing while the iPod was.