I've never done anything specifically like your project (I haven't had the request for voice overs). I am however doing some museum type work.
Off the top of my head... I'd create all of my sub-titles, make them invisible and then create a hotspot using the "deaf" icon to toggle all of the sub-titles on/off. You can find a version of this icon at: http://www.clker.com/clipart-43500.html
Depending upon cost factors I think it would be awesome to have video clips of someone "signing" the voice-overs.