Technical soaring ( the OSTIV magazine ) July 2002. pg 89.

The article suggests that the prone ( face down and forward ) position may
actually provide added safety rather than less safety when an analysis of
aircraft accidents is made.

However upward visibility would be a problem ( unless there was a horizontal
mirror in front of and below the pilots line of forward sight )
Keeping your head up could be fun after 8 hours!

Most of this is in reference to the Exulans ultra-light tailess sailplane.
http://members.tripod.com/~diomedes/exulans/

Ian