OK, so I gave it some thought and solved the problem in a reasonably neat way that involved no scripting. I'll try to explain how I did it in a straightforward way.
First, I figured out that you can set the default highlighted button for any menu by selecting the menu and then going to the palette and choosing 'Menu'.
Then I duplicated the main Menu and the special features Menu. (The special features Menu only has the commentary on it.) The duplicate menus were the 'On' menus -- Main-On and Features-On. Main-On linked to Features-On, and Features-On had the 'On' button set as the default. When you clicked 'On' it brought you back to the Main-On page. When you clicked 'Off' it brought you to the regular Main page, which would in turn bring you back to the regular Features page, where the commentary was selected as 'Off.' This neatly solved all the problems, but only when you're playing with it from within the menus.
But if you're viewing the movie and you select Menu, it'll bring you back to the regular menu, whether commentary is on or off. So I created two Stories, each of which is identical except that one brings you to the Main-On page when you click 'Menu' and the other one brings you to the Main-Off page. I set the Main-On page to play the On story, and the Main-Off page to play the Off story.
In this way, I covered all the bases and the commentary is always correctly highlighted in its present state. There may be an easier way, but this turned out not to be too difficult and it solved my problem 100%.
Good luck to all!