Hmm, playing the naive guy here; have you thought about approaching Valve+Nvidia after the Steam on Linux "thing" is official now?
Maybe there is a slight chance Valve can convince NVidia to provides som docs to you under at least NDA now.
I doubt this strange company will ever freely distribute it...
I don't think you can write open source drivers under NDA. It's how those devices work that is protected not the text, OSS driver discloses that information although it uses C instead of English.