This is the first time I've heard the Pentax described as small and light. I've never held one, but one day the local camera repair person went to the back of his shop (cave) and came back w/ one. My eyes bugged out. He tried to hand it to me but I wouldn't take it. It looked immense, and I am sure it is darned heavy, even when compared to your 'blad. The image quality is surely great, but a Hasselblad is about as big as I want to go for a walk around camera.
I second Krosya's idea of just putting a prism on your 'blad. I have one on mine, and while it adds a lot of bulk and weight to the camera the view through the thing is just incredible. But you've probably already thought of this, and nothing but the Pentax is going to do it for you. At the prices they're selling for, just buy one. If it turns out to be not your cup of tea, sell it and you should only take a small hit. Might even break even.