Can this camera do double duty as a normal camera (i.e. function as a normal D810) or will it only do a good job with astrophotography?
When shooting through a telescope, do you have the image circle and necessary focal length to even take advantage of the full frame sensor?
Also, I hope this is a special, on-the-side thing, rather than Nikon's answer to the 5Ds - that would be truly lame!
As mentioned you could use a filter and or custom white balances for daytime photography, but the extra IR sensitivity can be problematic to deal with for daylight images. The raw image will have a very definite red cast to it.
There are telescopes with a large image circle but they are premium type scopes, minimum $4K and up. An 8" f10 sct will not have a large enough image circle, you will need to use a high end refractor, like a Takahashi FSQ or similar or a 10" or greater mirror scope with a well corrected field, like a Planewave, or RC type telescope, and even some of those require field flatteners. A large mirror scope like a Planewave you are well north of $9k for optics. Then a mount for scope like that will run you $10K or more..... that is the high end of things.
You could do quite a bit with high end lenses and a small mount that was carefully aligned but for anything less than wide field you would need guiding for long exposures.
Astrophotography is very difficult, complex, expensive and at times one of the most frustrating types of photography and other times it is amazing to see what comes out of it. So many variables etc.
So for me a camera like this has limited use, it is a niche camera if you have a lot of money to blow or just like having extra toys. It will add some red sensitivity to details whether it is worth it for you depends on how much of that kind of photography you do.
Canon had two astronomy dslrs, the 20Da and 60Da. There are also a number of places that will convert other dslrs for astrophotography, either removing the Ir cut filter and replacing it with glass or adding a new cut off type filter. I don't know how many they really sold, I am sure it was more a niche offering. You see modified camera for sale quit often on Astromart.
Alan