why not let mirc use html colors
and even the option ( on/ off ) to display the other users their color.
Using html colors would have to be local only, otherwise only mirc would be able to understand it (unless other clients adopted it)
Staying with ctrl+k is the best way to 'upgrade' colors.. and the local colors only would prolly be the best route out of all of them.
I would think doing it this way would be less of a coding nightmare, it wouldn't change the already established 'protocol' and other clients would have the option to add more colors or not. This would also keep people who use other clients from seeing new mirc color codes as random giberish on their clients :p