So far the JBL is handling FAT32, with no problem.
Having a way for the book author to control the structure is certainly a good approach.
You might consider an output option to allow selection of a structure like you used for
the .pdb to .ePub conversion. I think such an option might address issues for other
devices, in addition to the JBL.
Many Thanks for your efforts and for taking the time to keep us informed.