I have 2 USB mass storage mp3 player: the Creative Muvo 100 and the Samsung YP-S5 with Asiatic v1.50 firmware. Rhythmbox has very simple support for those USB devices: simply add a ".is_audio_player" file in the root of the mp3 player (see: http://live.gnome.org/Rhythmbox/FAQ). The mp3 player is now automatically detected by Rhythmbox, and filling the player with songs from the FireFly server is as simple as drag'n'drop. The transfer is terribly slow, but that's probably because I'm connecting through wifi.
For both the Muvo 100 and the YP-S5 I put the following file in the root: .is_audio_player
audio_folders=Music/As the FAQ above mentions, this overrides the HAL. I didn't check (yet) if the default HAL settings are OK for the 2 devices. Anyway, this works fine for me.
Tip: with ctrl+H you can display the hidden files (=files that start with a dot '.') in Nautilus.