I didn't even know the word "backchannel" a year ago.
Last year during the presentations in Catholic Theology I had the students who were listening take notes. I was trying to encourage paying attention while their classmates presented, but at times it seemed that students were more focused on writing than active listening. They were almost transcribing, and copying down PowerPoint slides that were too wordy (despite my guidelines).
So, a bold experiment (for me). First, following Germaine's idea, no text on the slides. Just graphics, and the titles of their subtopics. And, I had the class on a chat website during the first presentation day. Write at least one comment, or ask a question, I said.
Some of the comments were simple affirmations ("I really like the slides" or "that story was AMAZING"), and while I had hoped for something a bit more substantial relating to content, I think the speakers probably appreciated those when they got a look. But there were some other comments that were closer to what I was looking for, and there were some good questions. It's possible that some of the good stuff came from a couple of students who are not usually very vocal in class. It also helped spark some of the conversation after the presentation.
I asked for reactions. They seemed to like the idea better than note-taking, anyway. One student said that it helped her to focus on what was being said. That is perhaps counter-intuitive. I used to think that the idea of a sort of conversation going on while someone was speaking was a bit... rude. So it was a bit of a leap to try it. I think we'll continue the experiment for the rest of the presentations.