funny, I just started to work on a similar setup a couple of days ago. I thought about attaching a HDMI-out camera to a big screen while the digiback is tethered to the computer. I tried some of the cameras I have in my cupboard at the moment
Go Pro Hero 3: doesn't work, it can't do close-up, it's fixed focal length, it's fisheye
Sony tx20: kind of ok, but no HDMI out
fuji x100s: wrong lens
Panasonic GH2 with a macro lens: too big, too heavy
Canon 5dmk3: too big, too heavy
Sony rx100: probably best, will inquire further
if you'd put a simple small hdmi-out camera onto a chimney finder and then onto a sliding back, all headaches regarding crappy live view for composition would vanish. make that usb-out to a simple viewer for your tether computer and it would be great!