That time Michelle from NSC told me as the camera was very new so there was no service charge on cleaning my D800, she did scare me though when she say that there were fungus on the viewfinder and apologizing later as what she saw was actually dust instead... lol...
We life's in dusty envelopment..
For the dust or fungus to stick on VF the round screen is very common problem.
For the external VF round shape can be screw out and using some microfiber cloth to clean it over & use the air blower to be clean it as well.
Even inside the VF and still got solution for clean on internal part.
The 2 small nos of screw out & remove the VF case & clean on internal plastic cover.
Also using the microfiber cloth for clean it over.
The internal mirror also can be using air blower to blow the dust out.
For the sensor you also can using Lens Pen & Strong LED touch light & blower needed.
For carefully clean it by own.
Just becareful of the oil base and may need take time to clean it slowly.
I also have this problem and not quite sure if I should tackle this job. Nikon doesn't give you the camera breakdown on how to manipulate the task. I'm in the Niagara region so not too far from nikon Mississauga for them to do it. Saw a video on a Canon of the procedure done on cleaning theirs just before I found you with the same prob. Let me know if you find out the (How To Do)