Nothing illegal, for sure ---
-- in that the device would be symmetric, no low-pressure side, no high pressure side. Any additional downforce would only come from the changing pitch of the chassis -- just like the car's entire body.
I didn't know the FIA outlawed it. The current SCCA regs surely don't seem to... and well, hmmm... I assume the FIA got rid of it to keep some last vestigae of the classic open wheel monoposto look...
The Vestal pic is a neat example of the idea; although I think the greater gain would be up front where the flow is generally less mangled.
I hope some of the aero folks will weigh in on this concept. I'm sure some study somewhere has been done.
Thanks for the thoughts.
C