There's an application that lets you lock individual applications, but which should allow you to unlock it with dictation, as the rest of the OS is still working. Mac App Blocker (http://knewsense.com/macappblocker/)

There's also QuickLock which was/is a workaround to lock OS X quickly without using the screen saver + immediate password requirement. http://www.quicklockapp.com/

Note: I haven't used either, I'm just googling and looking at videos.