Do you know how to do it or are you suggesting?
I ask this because of the screenshots...if you know how to do it please tell me!
Anyway I managed to make something similar: the dropdown menu shows only single-shot comics and parent folders (with a + in front of the name).
When you choose a parent folder, the menu will un-hide the corresponding subfolders (and the parent folder will get a - in front of the name).
Then, when you choose an already open parent folder, it will be closed (subfolders hidden and + in front of the name).
This is exactly what I wanted except for one thing: when you choose the parent folder the menu is closed and you have to open it again!
I can't do anything about this!
PS the best way would be understanding how kindle's menu are created and replicate it populating it with parent folders and single-shot comics....
BUT, personally, I don't know how to do it!