I’ve actually been just using the default color mixer under the advanced color selector for this purpose. The artist color mixer also has most of the functionality, but to be honest, I was thinking of writing a plugin like this for a little while now.
Still, to be useful for me, this would need to have custom color blend algorithms, because I use the above color mixer plugin specifically for the different algos. Being stuck with basic srgb mixing has never been fun on CSP, and it would be even more limiting in Krita — if I can make a canvas that is just a palette, with better color blends, it would make this addition obsolete right off the bat.