Restrict Permalink Categories

I have a custom permalink structure /%category%/%postname%.html. My posts have more than 1 category, but I would like the permalink to be created from 1 subset of categories and not the other. How would I hack WP so that I can restrict the creation of permalinks to a certain set of categories (or sub-categories)?

For instance, let’s say I want permalinks to be created from children of cat id 26 only. How would I do this? Thanks!