As a workaround i've setup Chrome Remote Desktop at any PC i use, so I can easily connect to them without much network hassle and turn off stuff like Google Music.
Seems like a pop-up implementing this would be ideal, since it's most often accidental: "Hey, this is still running on another device, and you're only allowed 1 device at a time - do you want me to remotely kill the other device's Google Music session?"
After all, most of the time it's not another human using both sessions at once.
