This works... You need to be rooted and install script manager

Use this as  a script

mount -o bind sdcard2/android/data/com.google.android.music sdcard/android/data/com.google.android.music
Alternately you can change the first location to the location of your choosing as long as it resides on the external as card.

Ex.
Mount - o bind sdcard2/music sdcard/Android/data/com.google.Android.music

Go to google music (play music now) settings and clear any music that is currently saved to your device.

this only applies to music made available offline. If you do not clear it before running the script the music will exist on your internal as card but not be accessible.

open script manager and find the script you made
 
Select it and make sure to click set on boot and superuser

Run Your script.

That is it now your default save location for google music is bound to the location on the external sdcard. the device will see the location as its default but really be saving to the external sdcard.

You can check this by going to google music. Selecting "make available offline" and look at the free space. It should shoe the space available on your external sdcard.