This has been asked on
FBReader's google group, look for the thread "Does FBReader support Nuut or Cybook ebook devices?". The generic answer is that first there has to be a SDK, and single-purpose devices don't typically have one (even if they are based on Linux).
The option of wiping Bookeen's software are starting over is interesting. It would be a lot of work, but Bookeen will presumably have to make its Linux software available to everyone - so it might be possible. The same approach should be possible on the Sony.