ive been thinking about this for along time also. my thought was to somehow bring the air in through the rear windows (probably a larger duct than the NACA style ones) then onto the rad, for the air to exit I was going to modify the rear light clusters (maybe drilling them to death with loads of 10mm holes or removing the majority of the lense completely) either side. The rad obviously pretty much enclosed in a carbon/fibreglass shroud so any air that is brought in throught the vents goes over the rad. Then some serious big ass fans behind to force air flow should the car be sat still, as its not in the engine bay and there is plenty of room some big fans shouldnt be a problem. How these would effect air flow in normal driving im not sure but considering all our rads now currently have a fan and intercooler right in front of them I wouldnt think it too much of a restriction? As long as the cut outs in the lights could flow as much as the window inlets flow I reckoned on it been a reasonable system and not too permanent like cutting holes in the boot floor that might not work. Just my 2p worth and im in no way an air flow engineer nor a cooling engineer so its probably riddled with flaws.