What about a 650b conversion on a bike with abnormally low BB?
The only bikes coming to mind with abnormally low bottom brackets are DH frames - my old 2000 M-1, for example. Even with 165mm cranks I still smacked the pedals often. The that bike cornered like you wouldn't believe. The downside is weight (10#+ frame), single chain ring only, and the need for a min 7" travel fork. Not a great xc bike, although it was fun on flat trails once in awhile.
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