Well, you could treat 'em like data: drag the song files onto the iPod in Windows Explorer, then when you get home, drag 'em from the iPod in the Finder into iTunes. Then delete from the iPod (in the Finder again) and sync.
They wouldn't be playable on the iPod until after you do your thing on the Mac, but it'd still work in the long run.
Of course, there are several Mac apps for moving stuff back and forth between your iPod and any ol' iTunes library (iPodRip, senuti, more whose names escape me). Somebody's probably figured it out on Windows as well..