Most all of my music is ripped as Apple Lossless for archival purposes. But then due to iPod space, I re-rip as MP3.

I dearly want to sometimes make copies/backups of only the MP3 part. Of course, I could go into every single sub-folder and select only the MP3s, but that would be super tedious. (Please don't just say get a bigger backup drive and back up everything; one application is to SD or flash for playback in a car, and to a Rhapsody player in future, and from my Mac to a small-HD used Windows laptop I have).

What I really want to do is somehow copy all the relevant folders (say "White Stripes" or "iTunes Music" but have only the MP3s actually copy. I'm imagining via drag-and-drop, but could be some other way.

Or, somehow "burning" (actually exporting) my MP3 smart playlist...

Any ideas?

Use a terminal command:

find /Users/someone/Music -iname '*.mp3' -exec cp '{}' somefolder \;

The command above will copy all files in /Users/someone/Music that end in .mp3 to the folder 'somefolder'. ("Automator" would also do the trick too, if you're more familiar with that.)