You might check out the RECUMBENT CYCLIST NEWS (RCN).
http://www.recumbentcyclistnews.com/
The July/August 2000 issue had an article titled "Numb Feet, Recumbents & **** Erectus" by Patrick Quillin PHD, RD, CNS (whatever all those initials mean). He basically said that some recumbent riders get numb feet from a lack of circulation caused by elevating the feet above the hips. Bob Bryant, editor of the RCN has written that he has the same problem on some recumbents which have a bottom bracket higher than the seat.
Gary