There are still a few rear-engined sports racing classes about which do not allow raised diffusers (as per the old SR2 rules) but do allow flat floors under the cars.

If you look at any of the old SR2s or the more modern cars they either
a) have nothing rearward of the chassis; or
b) extend the flat floor rearward (usually to the rear axle line) and then abruptly stop

It occured to me the other day that if you took the flat floor concept but cut out two NACA shaped profiles either side of the drive train (roughly speaking it would look like an 'M' from above) then it may be possible that the airflow under the car would fall over the edges of the cut out creating a vortex (as per the standard NACA duct mechanism) and in doing so lower the pressure reaward of the chassis floor and help accelerate more air under the car (i.e. do the same as a diffuser is designed to do).

Obviously you cloudn't use the rest of the NACA duct as it would effectively be ground effects tunnels - so only the opening shape

I suspect that if it would work then someone would already have done it and that if it really did set up the vorticies it would add significant drag but does anyone have an opinion as to whether this theoretically could serve a similar role to a raised diffuser?
(before I go and get out a jigsaw and some ply wood!)