There Is a plug-in, ml_iPod, that has an option to apply mp3gain to files before transfering. Although it was designed for the iPod, it will work with a USB drive. Read the wiki.
http://mlipod.sourceforge.net/
http://mlipod.sourceforge.net/wiki/I..._for_all_songs
http://mlipod.sourceforge.net/wiki/U...SB_MP3_players