kartu: I've looked at flipping the screen outside of tinyhttp. Interaction with the driver tells me that the flip succeeded, but when control comes back to tinyhttp it resets back to the normal orientation. Sorry. Here's the code and executable.
The log it produces:
Attempting to access the framebuffer device
setting power mode result: 0
Power mode: 1
EDIT: I also edited ebook.so to play around with the orientation, but I'm not sure if those are the functions called (EinkSetOrientation and ebookEinkDoRotate) by Fsk. I only tried so far to restart tinyhttp with LD_PRELOAD, but that didn't work. Might need to do a proper mount --bind, but since I need to go sleep, I'll give the .so to you and maybe you'll have better luck than me.