That's an interesting use-case. I haven't had to worry about that yet.
Since it's options being updated, I would throw together a script that hooks into update_option which then determines if the "master" site is updating its options. I'd probably put in a few more checks to make sure the updating isn't coming from some page I want to be unique on a per site basis, then go through the site option tables and update the value for each one.
That's my initial thought. The biggest thing you'd need to think through with this method is making sure you're not updating what you shouldn't (i.e. General settings), and ask yourself the question of what happens when the user updates something for the sub-site. Should they be able to? If not how do you communicate that? If so how do you preserve settings that they've made?
Fun little mental experiment.