I am checking to see if g_file_storage gadget (USB mass storage driver) can export RAM disks as drive letters. That would be better in multiple ways (not just preventing corruption to the userstore partition). If both sides have (rw) access, it would not matter as long as we limit our writes to a pre-defined file for each side (and mount "noatime,nodiratime" on the kindle to prevent writes to partition directory structure). One fixed-size file for the host PC to write to and one fixed-sized file for the kindle to write to should work.
Another option is to skip USB mass storage, unmount both the drives exported by g_file_storage.ko and the zero-byte drives (for charging) exported by g_zero.ko, and use
hidapi for USB HID data transfers (on both sides of the USB connection).
In any case, I will eventually succeed (or get distracted into some other project).
