If you missed this the first time through, like I did, you can use the "Developer Commentary" option (on the Single Player menu after you beat the single player game) to select "Chapter 7 - Reunion" Yeah, you'll have to play through the first part of the chapter, but at least you don't have to start over (if you don't have a save point).
BTW, I just used this method on Chapter 2 for the "Final Transmission" achievement and the achievement did unlock while in Developer Commentary mode. If you find this does not work for this achievement, please let me know and I'll take the solution down.