...... If Nikon would get off their butts, what I probably really need is just a 50 Mpx sensor with larger photosites and I would be happy.
I'm sort of confused by this comment.
If it's a 50Mp MF (or larger format) camera you need, then why not just get one!?
That's basically what you're wanting here .. a greater amount of pixels AND larger photosites .. from Nikon, who currently make cameras no larger than 135 format!
There's no reason to expect that they will ever do so .. and even if they did, it'd be hideously expensive(eg. compared to the Pentax 645z!)
So just do yourself the favour and get a 645z!