just in case anyone needs this later, here's my workaround:
STRAWBERRY_PID=$(ps -C strawberry -o pid= 2>/dev/null | sed -e 's/ //g')
ls -l "/proc/${STRAWBERRY_PID}/fd" | grep "<path to my music>"
so I'm just getting the file descriptor of an open file in my music storage, opened by the strawberry process.
not nice, but working, at least for files (not for streaming).