Yes, they exist.. thanks in part to Staton-Inc..
http://www.staton-inc.com/
..mid-drive unit..
http://www.staton-inc.com/Details.asp?ProductID=3252
..chime in here, if you might have useful insight to share..
http://atcnuvinci.informe.com/viewtopic.php?p=202#202
..to quote Reid, ".. let's...