After reading about philip1's screened belly pan, it occurred to me that there is a significant advantage in that it minimally restricts airflow from and to places where it really needs to be. Connecting the dots, it occurred to me that rally racers have been using a similar concept for many years with flexible underbody flaps.
I had always assumed that the only real purpose of these flaps is to deflect debris away from the undercarriage, mitigating damage and buildup of materials in obscure underpinnings, but it seems pretty obvious that there should be an aerodynamic advantage as well. My theory is that such flaps, albeit likely on a shorter scale that does not drag on the ground at slow speed, provide the best of both worlds: Cleaner underbody aerodynamics without restricting airflow in any way, while allowing relatively unfettered access to everything for servicing.
Anyway, I think this is something I will likely try at some point, but I'm interested to know if anybody here has any experience with this sort of setup or can point to testing to support or refute the supposed benefit to aero.
If not, I'll update this thread at a later date when I get around to my own testing.