The human eye can't make the difference between a "broad spectrum" and a simpler spectrum obtained by addition of three colors and that leads to the same RGB components.
There are however some colors that can't be obtained by addition of three basic colors.
This is the case for example of all the pure monochromatic colors, except the three basic colors that would have been chosen.
The set of those unattainable colors is rather small if the three basic colors are suitably chosen. RGB is a good choice that leaves only a small area of unattainable colors.
Note finally that the camera can't record the whole spectrum of colors either.
Cameras also record colors, like the eye, as additions of three basic colors.
Therefore, using "natural light" is very unlikely to bring anything to CinemaScope.
Furthermore, it si totally possible, and even rather easy, to create a light source with a broad spectrum close to the spectrum of natural light. It is even possible to have an artificial spectrum larger, even much larger, than natural light.
It essentially depends on the temperature of the source ("color temperature").
Sources at about 6500°C have a spectrum close to natural light. (the material used doesn't even have much influence)
http://en.wikipedia.org/wiki/CIE_1931_color_space
http://en.wikipedia.org/wiki/Color_temperature