Is it the software manufacturers or is it the design of the Pentax 645D?
A bit of both. Pentax did not really bother with tethering the 645D. Obviously is is possible. I don't think the software folks are interested in investing the time and money to do it.
But if I am not mistaken, can't you automatically open a new image in LR? So transferring an image using the new software or EyeFi card would have the image open in LR. You could try an EyeFi card or the new software with a camera you already have to see if it is possible.
If you want to tether to C1, then you need a Phase/Leaf camera. There are 22MP Mamiya/Phase cameras for about the same price as the 645D. The sensor is a little larger as well. That should work really well in the studio.
BTW, I use two manual focus film lenses with the 645D and I have no problem focusing them in the viewfinder--A 35mm and A 120mm macro. You can also check focus after the image is taken in the camera.