I read this and first said, 'this is an issue?' So, I got out my Z and tried it myself. I kinda see why you are asking now.
So, what I automatically find myself doing is to 1) put the LCD of the camera against my chest, 2) grip the lens to be removed in my left hand (mostly, but I tend to support the camera a bit too with my left hand, 3) using my right middle or ring finger, I push the lens release and turn the lens to the right, and off it comes.
I suppose it depends to on the size of your hands but that's what I do. When it's on a tripod, I recall I tend to remove it from the front of the camera, right hand turning the lens, left pushing the release button.
Hope that helps. Honestly, it's essentially what I do with my k mount cameras as well. Now if you were asking about the Pentax 6x7...now that's a different story and is a handful.
Afterthought, I don't think it really matters whether the lens is a 645 A, FA, DFA lens (or as I first had to remove a 6x7 lens with a 6x7 to 645 convertor on it, same process) with this either. Ok, I'll grant you if you are rocking say the enormous 645 FA 600 lens, you might need a slightly different process...just sayin'