Without wanting to jump the gun and accounce this project as a success before it's got started, I just had a nice affirmation that self voicing is useful.
I wanted to take some screenshots to illustrate accessibility problems for another post to this blog, but I was dealing with some other applications and still had my headphones on with my self-voicing SL viewer was running in the background with my avatar at the start location on Orientation Island. This is clearly a busy thoroughfare, so it's perhaps not surprising that after a while someone would try to chat to me. What did surprise me was when my viewer spoke the chat text and interrupted me from my other work.
I'd only tested it with bots and alts previously which is a bit contrived. This occasion was less of a technical affirmation and more of a social one - the viewer alerted me to something that needed my response similar to the way instant message clients use audio to alert the user. In that sense it worked: someone in SL wrote me a message and wanted me to respond. Although I was AFK for the purposes of that application I was still able to keep my presence 'alive' even without needing to see the chat message. The same thing happened a little later with a friendship notification.
Also interesting to reflect on the adage "One person's accessibility issue is another's usability issue." I am ostensibly working on an accessibility project, but it turns out to have positive usability results.