I did some night parade stuff with my D800s during Christmas and simply cranked up the ISO to 6400 and 12800. During pixel-peeping, they seem marginal, but with good processing of the raw files with ACR and certain images with Nik Define, I have a usable edit. When I travel to Serbia and the Balkans, I'm in a similar situation regarding dark monasteries, slow lenses and marginal conditions.
Normally for low light and this kind of work, I prefer shooting with my D3s which is nice, even at 12,800 and 25,000. I've gone higher when I've needed to do it.
I've been pushing the ISO for my kind of shooting now for several years.
The D700/D3 are fine up to 6400. The D3s gives me nice images at 12,000 and useable at 25,000. It's a matter of an image vs. no image in some cases. When I went with the D800 three years ago, at similar sizes (down-rezzed) to the D3s, it makes the grade. Pegged at 25,000 the D800 is a better image quality than the D700 by far.
For the most part, most images made pegging the ISO on the D800 are not necessarily useable at the full resolution for most images. However, up to say 11x17, all these cameras from the past several years are just phenomenal.
With good capture technique, good craft and good post processing, you can get good results. Part of it requires practice, part choosing just the best of the bad lighting situation.
I come from years of film where color above ISO 64 was hit-and-miss and Tri-X at 1600 was the state of the art. With the D800, I am fairly happy up to ISO 6400. Higher simply takes more post and the quality starts to suffer but is still doable.
Since the D800D810 has pixels to burn and most likely you are not doing 16x24 and larger images, simply keep pushing your camera and your craft and see how far you can go. I can tell you that of my last couple of museum shows, no one was the wiser on the photos shot at ISO 6400 nor even the 16x20s printed from an iPhone.
It all boils down to choosing the best images and then using good craft to make them shine!