I promised an update, so: I thought The Open Book was a pretty smashing success. Just about every attendee wrote in it. The resulting reading is inspiring (though I can't say much for the handwriting... pick a wide-ruled notebook if you can!).
A little contrary to my expectations, more people felt comfortable committing to actions than talking about how open access has benefited them. Unexpected, but hardly displeasing! I think it helped that I emphasized that the action they committed to didn't have to be huge, and priming the book with my own story and promise also made a difference. Most people committed to relatively small things—and that's fine. Commitment is commitment!
I will do this again at other events, and I now feel comfortable recommending the idea to others!