On further thought, two problems with the fan guess:
1. Wouldn't the fan have to reverse direction with the bellow changes? The air flowing into the the accordion would turn the fan one way, air flowing out would turn the fan the opposite way. Seems way too clumsy to me.
2. The draw notes "suck in air" from outside the accordion and would not be as affected as the push notes which uses air inside the bellow.
I'm sticking with the "hidden reed block" theory with the back stop operating airflow into the off-tuned reed set.