[justme1969]

It is a wonderfull idea in that the perforations will create more air cushion at any transition area. Now window screen may not be a good choice but screen with perhaps 20% - 35% perforated surface on Kamm back should do wonders in dispersing the high pressure stream and low pressure pockett that results from transition.
Most rear spoilers are raised for similar effect allowing low pressure pocket to be blended into high pressure over flowing stream.
Exception would be if compressing rear end for down force sticking surface into high pressure stream is key and even more turbulence is created.
This is why most OEM trunk spoilers are just for looks and hurt performance in both categories.

I ment that most spoilers are raised allowing low pressure air to be vaccumed into overflowing stream from under them reducing low pressure pockett signature.