With the blue area, it appears the attachment could be made to the bottom bumper. The idea of drilling into the painted area is unacceptable. Covering the air vent may be discouraged.
If the plate needs to be above the air vent, then a floating bracket that does not touch the paint would make sense.
The area under the front vent looks promising as a low mounted plate.
Much depends on the placement of the radar unit(s). If on the side, then it shouldn't be an issue. If the radar is in the center as with Model S, then the plate should be mounted above the blue boxed area. We need to wait for the mounting plate and see where Tesla Motors suggests to mount the plate.