I personally cannot think of a reason why your CDK shouldn't work very well as a planetary scope, all you'd need is a 3-5x barlow depending on the pixel size of your high speed camera. I imagine that a lot of planetary images aren't taken with the CDK because it is over engineered to some degree. An 18" newt would be cheaper than the CDK I imagine, for planetary a large corrected imaging circle isn't needed as a small sensor is all that is required.
Hell, for the price of your CDK and mount you could probably by something on the order of a 25" tracking dob which could achieve the same result
Basically, I think it is simply that the people buying a CDK are more interested in galaxies than planets.