Here's a klunky workaround I'm using:
https://www.mobileread.com/forums/showthread.php?t=7870
Hope they fix this in next firmware rev. Ideally I'd like to see the reader automatically put FAT folders into a "Collection" on the book. And also support for nested collections
I did think about a bunch of cheap 32 MB cards in a Altoids gum box, but then you've got that to keep track of.