Thanks so much for all the thoughts and research! What I ended up doing (and hope it's ok) is to have a TN that says that the number of holes is not represented in braille, and that the student should ask her/his teacher for help. Then I did a two-page graphic with all the buttons in their shapes and rough sizes, and approximate positions on the page. And I textured for the three colors. With the key, it took up two pages total. And I'm kind of limited to two pages, given the size of the volume.
Again, thank you so much!