I think this will work, providing the Cybook does not draw too much power. Note that you don't even need to rewire a connector see:
USB OTG the easy way.
The N800's web browser should be good enough to navigate any of the MOBI web sites, so it can be used to download MOBI files for the Cybook. DRM-free MOBI files will work on the N800 (FBReader) and Cybook.