I was working on the car today, and I noticed something very interesting. The underside of the car slopes upward, starting near the gas tank. 25" from the end of the car (in line with the wheel centers), there is 7" ground clearance. 4" from the back of the car, where the boattail would need to begin, there is 10" ground clearance. That gives an overall angle of 8° in this region.
I don't know if a stock Insight has attached flow in this region, or if it could have attached flow with an undertray installed upwind. But it does seem plausible. I would tuft-test it, but I don't have a webcam or an undertray.
I just watched the footage of MetroMPG's tuft testing of the underside of his boat tail, which starts at 5:00 at