There's always more than one way to skin a cat when it comes to a powerful o/s like GNU/Linux.
You can try this within a directory of mp3's at the command prompt:
ls -1 *.mp3 | sed "s/\(.*\)\.mp3/\1.mp3 \1_128.mp3/" |xargs -n 2 lame -h
Notes:
1) I haven't tested it so you might want to test it in a directory where you have just a few mp3's first to see that it works
2) It should output new mp3's with filenames like: song_128.mp3
3) This will only work if the mp3 filenames have no spaces in them
4) The meta data will not be carried over to the new 128 files, but neither would that happen with the way you're running lame in your example.
I'd like to know if it worked for you.
Cheers