Is it possible to partially hex vertex sets?
I mean, if two parts of the model are on the same polygon, could you swap/remove only so many vertices by hexing?
Well, if you wanted to partially hex a polygon, just don't change all of the hexes to 0. Just change some of them to 0.
But the reason no one has even bothered doing this is because you can't even see what you're doing and no one knows with hex is which vertice.
So my answer is yes, but it would be hard to do and would take months to figure out.
And I'm pretty damn sure no one wants to waste their life figuring out how to do this when they could just vertex it out... 
