Hasselblad made the "Hasselblad Linear Mirror Unit" which shows up every now and then on eBay. It's best used at shorter distances (under 4 feet) from my limited tests to date.
Shooting straight down can usually be squared up using a good level. (Remember aligning an enlarger?) Maybe buy an extra camera body and setup a permanent copy stand.
Archeologists have used a long stick with a knotted string attached to the ends of the stick to triangulate centering and aligning a camera to a subject in the field.
There are a few advantages to using a Hasselblad. You only need to align the camera and not each standard of a highly adjustable view-camera. Although I would expect the Betterlight to produce the best images, you won't need to pound the subject with light to use f11-16 and therefore getting a decent focus will be easier. (Not fun trying to get the focus across a large copy when you're stuck shooting at f5.6)
Hope some of this helps...
Chris
