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.
After all, most of the time it's not another human using both sessions at once.