A slightly open trunk would alter airflow, and would certainly bounce around. I don't think you'd be able to get any sort of consistent readings.
The purpose of a wing in back is to help plant the rear tires; the more severe the angle, the more downforce. But the penalty is drag. Any race car will run faster without a wing up until the point that the rear wheels begin to lose traction. Not many of us need worry about going fast enough for any of this to matter. Overdrive the car and you can certainly lose the back end in a curve at street legal speeds, but at those speeds our little decorative wing doesn't make one bit of difference. An enormous, steeply raked World of Outlaws wing might. But probably not.
dcb