EDIT: Whoops, I see you're referring to slant parallelograms. When I wrote this post, I was thinking of turning a straight parallelogram into a drop parallelogram. Disregard.

René Herse already did it - it's on his 1980 Tandem Chanteloup as shown in The Golden Age of Handbuilt Bicycles. The hanger is a Herse-fabricated piece which sits lower than a conventional hanger. I believe with the derailer stop was rotated 40 degrees or so, counterclockwise.

This made a straight-parallelogram rear derailer sit in the same location as a drop-parallelogram; in the case of this 1980 Herse, the RD is a Simplex Super LJ 5000 long-cage.

I'm not sure if this is the same '80 Chanteloup, but you can just barely make out :

(Those of you with the book will see a better photo of this RD setup on page #158 and #161; the 1962 Chanteloup on page #160 has a much shorter hanger with a Huret Alivit running in the conventional manner).


-Kurt