During one of my journeys through the depths of the internet I just stumbled upon this archived page from the Spanish GW website, which contains a lot of cool fluff about the Navis Nobilite, including the personal background of one particular Navigator (and his Naval Bosun bodyguard) as an example. The political situation and hierarchy are discussed, as are potential scenarios for Navigator characters.
I'm not sure if everything is compatible to the background in FFG's books, but I am sure that even should any contradictions arise it could still be useful as inspiration for players interested in a Navigator character. Maybe someone will find it of use.
Oh, and also check out the page about Xenos - ever thought about ways of having a Tau or Eldar character around? Well here you go!