What you describe, editing saturation curves in RGB mode, may not be possible. However, it is possible to enhance a few colors in an image with the curves tool.

The way I think of saturation, which may be incomplete or even just plain wrong, is decreased saturation is more gray, while increased saturation is less gray.

In RGB, the gray tones have an even amount of each of red, green, and blue. This is often used to "white balance" an image. Several points are selected that are intended to be neutral. Curves are adjusted to make the RGB values approximately equal. Do this through out the image at various intensities to remove color shifts.

If, instead of neutralizing colors, you want to emphasize colors, you can select areas of an image you want to enhance. Then adjusting the curves so that the RGB values are farther from neutral. It helps to know the RGB values of some of your favorite colors, as well as the inverse of RGB: Red-Cyan, Green-Magenta, Blue-Yellow.

For example, to emphasize "blue" tones in part of an image, select a few points. Then for those points, reduce red a bit, maybe leave green alone, and boost blue. Since you likely don't want to give the entire image a cyan-blue cast, make small, limited changes to the curves around the regions of the selected points.

Although enhancements to multiple colors can be done within the same set of curves, it's easier to work on each color in its own set of curves. When using curves this way, I usually use color blending modes to avoid letting luminosity get out of hand.