It is more advantageous for the CTB-er to utilize the smallest space possible, and one that is as tightly sealed as possible. Sure, one can die by "low" levels of CO over a long period of time, but, for most, that will not be desireable. There are many unpleasant effects that one experiences from CO before death occurs. Most people desire to avoid these effects, at least to not be consciously aware of them. The only way to achieve that is to fall unconscious quickly. And the only way to fall unconscious quickly is by creating an environment in which the CO level is high. A level of CO within a given environment capable of rendering someone unconscious quickly is, at least, 10000 PPM. It only stands to reason that utilizing a smaller space (volume), that is as tightly sealed as possible, is the most efficient way to achieve such an environment. The lower the level of CO in a given space, the longer someone will have to endure the undesireable effects from the CO until, finally, falling unconscious.
Could work. How much charcoal, and what type, are you planning on using? And where/how would you place the very hot grill in the car? You'd really need to seal any potential leakage points in the vehicle in order to achieve, and maintain, a high-level CO environment, which is conducive to achieving the most peaceful death possible.