One reason I bought a flatbed scanner was so I can scan my 120 format
6x6cm negatives, none of which are cut into segments. But the design
of the supplied adaptor apparently requires one to use cut up
segments, as there are a few small holes in the adaptor that the
scanner uses for some sort of calibration or orientation process. When
these are covered up by the film strip, the scans come out with dark
streaks on them, or the scanner doesn't detect anything at all.

How about this product? http://www.betterscanning.com/

I think I remember reading that there is a 3rd party adaptor for sale
somewhere that allows one to scan 120 format negatives without cutting
them into parts, but I can't remember for sure.