The problem is Google play music only looks in the music directory for songs.
Also you cannot create symlinks in the fused sdcard.
You could try the following in a terminal emulator.
(Note you may have to change the following paths as I can't remember the exact path stickmount mounts your device to)
mount -o bind /sdcard/music/sd /sdcard/usbdisk/sda1
This should create a folder called sd in the music folder which Google play music is looking in. It should be then linked to your mounted storage in stick mount.
But beware this may crash your phone when you remove the card requiring a reboot.
Sent from my Nexus 5 using xda app-developers app