Copy contents of a smart folder to a new folder in Mountain Lion
You could use mdfind in a shell script to simulate your smart folder's settings (mdfind is how you use Spotlight from the command line). Use kMDItemKind to specify the kind and kMDItemAudioBitRate for the bitrate.

Then you can use that list of files and copy them to your external drive.

Something like this script below. (scp copies files to a server; use cp to copy to a drive. Or, if you want to get fancy, rsync is generally better for incremental backups.)

#!/bin/bash

for FILE in $(mdfind 'kMDItemAudioBitRate &gt;= "44000"')
do
    scp $FILE myname@myserver:~/music-backup
done