I also would really like the ability to paint spoilers and mirrors.
It is possible to cover the mirror if you apply a decal to the side of the car, position it just below the mirror and then move upwards until you see it overlap the mirror. I generally use a circle and start off small, then increase the size as necessary. It can be fiddly though, depending on the shape of the mirror.
However I see no way at all of applying a decal to a spoiler. The only thing I can think of is to paint the car in the colour you want for the spolier, then cover the car in decals of the other colour you require. This would be pretty messy though and results may vary for different cars.
The thing I don't get is that there are plenty of pre-set wraps that feature painted spoilers, mirrors and other details so it must be possible.
