It seems the biggest gripe about rangefinders (vs SLRs) is that your shots are not quite what you see, there is a parallax error due to not seeing through the lens.
Which makes me think, if you put the rangefinder viewport directly above the lens it would eliminate horizontal parallax. This would be nice.
Is it just the fact that the lens blocks some of the view and you want to have the viewport far enough away to avoid this, or is there a physical issue too with how the rangefinder functions?