The RS20's CMS clearly has issues, but I seriously doubt that the problem is simply a matter of the color space they used when engineering the solution. Sharp's CMS works in the HSV color space, which is very similar to HSL, and it works flawlessly. Also, the Epson CMS appears to operate in HSL and it works fine as well. I actually think that the problem with the RS20 CMS is more serious than Jim suggests.
It appears to me that the problem lies in the fact that the "saturation" control isn't really that at all, but simply a chroma gain circuit operating independently over each of the pri/sec. colors. That, along with the insufficient range of adjustment (which suggests a limitation in the hardware), is most of what you need to explain its deficiencies.