In Together or Apart: Collaboration Models for Technical Writing, Tom Johnson reflects on using a book sprint for producing technical documentation and in community projects. This would be an interesting approach for producing an open textbook at a conference. Get a group of people together to come to the conference a day early, prepare a draft in the pre-conference book workshop, and share it during the conference for feedback. Then stay an extra day, incorporate the feedback, and prepare a final draft.

Obviously, it would be difficult to write a textbook from scratch unless there were a lot of people and a lot of small chapters. But it could be a great method of taking a Wikibook or set of other resources and "remixing" them into a fuller, more polished text.