I had the opposite problem (many keywords marked for export that shouldn't have been) and came to the conclusion that the only supported way to fix this was with the SDK. See http://www.lightroomforums.net/showthread.php?12537-managing-quot-export-quot-setting-of-many-keywords.

Taking someone's advice on that forum, I used the SDK to write a small plugin to do this.
For my case, if a particular top-level keyword is not exported, then all of its children (and their children, etc.) should not be exported. However, if the top-level keyword is exported, its children shouldn't necessarily be, but I wanted to know where that wasn't the case. I wrote about this in more detail here. You can also find the code at the above forum link. If you're willing to hack around with Lua, it shouldn't be hard to modify my code to do what you want, but it has no UI except a menu item that runs it, so you really would have to dive into the code to do anything at all different from what I did.