Modify Texture at run-time. Modify, not change.

I want to modify a texture while the program is running, it’s like this:

I want to put a sticker at a car, and the player can choose wherever to put it in the car, but the thing is, the car has the sides and etc, so I can’t just put it on top, because if the person moves the camera it’s not “stuck” on the car, but on top of it and in the borders you’ll be able to see it.

I want to merge that object into a run-time texture created through code and put it on the car, is that possible?

http://unity3d.com/support/documentation/ScriptReference/Texture2D.SetPixels.html