One of the co-owners of the gallery i joined - had a D800 with 36mp. Complained to me that it gave him fuzzy pictures so he bought the D750 24mp camera when it came out. Gave the D800 to his painter wife
He's now very happy with his Nikon FF.
I've read that the D800 needs the best technique to get good pics out of it. It's also my impression that the D750 has been a very popular camera with the Nikon crowd. I assume that popularity of a 24mp FF is for a good reason. So perhaps a 36mp sensor has its drawbacks as well.
So my point is - a 24mp FF sensor might be a better choice for a FF camera. The sensor should be cheaper than a 36mp FF sensor; it will still have superior DR and low light performance to a K3. The type of individual that will buy a Pentax FF will most likely know that there's more to a sensor than the MP count.
Finally, here's a DXO comparison between the K3, D750 and the D810.
Nikon D750 versus Nikon D810 versus Pentax K-3 - Side by side camera comparison - DxOMark
So the D750 with 24mp is superior in DR to the D810 and far superior to a K3. So a person who thinks a 24mp isn't superior to a 24mp K3, probably doesn't deserve one :-)