Update: sshdroid now works well for me with scp. Be sure to change the default password!
Wow, what a pain. MTP support in lucid is broken, or at least incredibly slow for galaxy nexus. PTP works okay with nautilus / gphoto2, but is also painfully slow. The various ssh servers all seem to be broken (I just get connection timed out errors when I try to ssh in).
Ultimately I ended up installing the "SwiFTP" ftp server app (ugh! plain ftp with no security!), but it worked great over wifi. I even managed to mount it using curlftpfs as root:
curlftpfs -o allow_other,user=username:password ftp://firstname.lastname@example.org:2121 /mnt/GalaxyNexus
With that done, I could use rsync to copy some music over:
rsync -T /tmp -rv --progress --exclude='*.wav' --exclude='*.flac' . /mnt/GalaxyNexus/sdcard/Pictures/music/
(The -T /tmp was necessary since curlftpfs doesn't seem to allow creating temp files)