The K5 demo successfully exports the USB drive from the diags menu. The demo contains a series of numbered .gif images, and a simple config file that defines touch zones for each page, and which page to go to when touched there. It has comments to support swipes and pinch and hold, but only taps are used in this demo.
The USB drive exported when booted main is 0MB, so you need MfgTool to get into diags.
It appears to be a "standard" diags partition. I will compare it after I copy my "secret" jailbreak to the exported USB drive.
But first, sleep.
P.S. As mentioned before, the demo K4 already supports USBnet in its diags menu (also like stock diags). I want to get unmodified partition images from the demo units, for comparison to normal partition images.
UPDATE: My "secret" method still works. I exported the USB drive from diags on my demo K5, then manually created the files, then launched it from diags, and it displayed "Hello world" on the eink display. So I can modify that to make partition backups, and even to install the missing SSH files without needing fastboot or a pre-hacked diags partition image. I am waiting for my paperwhite so I can try it there too.