Google Music Offline Tracks
Finding out where it's stored is easy. There's a terrific little app called DiskUsage that visualizes the space your folders take up. You can easily spot where the Music cache is. 

As for moving to a new rom. Based eldarerathis' answer to your previous question, you should be able to make a backup of Play Music's music.db either from a terminal or using an app like Titanium Backup. Restoring this file on the new rom should make cached music show up.