CGoodson, some things you might think about.
Have you REALLY checked HANS to see if they can't accomodate an extreme lay-down vehicle.
Look at Scott Sharp's crash in the Patron American LeMans car last week . Total destruction! He walked away and ran the race with a backup. He wore a HANS device.
All Formula One drivers wear HANS devices.
The late Ed Rannberg who drove several lay-down vehicles during his fifty year career, including the electric streamliner that holds the FIA and BNI Class Two records, told me on many occasions a driver/rider MUST be able to have his face forward to adequately control the vehicle. (MAYBE you're gonna use a periscope, then you're on yer own).
Build something to drive/ride around and practice control,
then draw your own conclusions.
Well intentioned comments. Good luck.