Jim, welcome to my world

I, too, found that's it's really hard to get any feedback, let alone the usable variety. And apparently we're not alone (Scott Berkun has a few choice words on how "the feedback loop for speakers is broken" in his book, for example).
The direct approach, i.e. simply asking people, often doesn't work, as you found. They are too polite to tell you that you sucked - or they don't want to admit that they couldn't follow you. It seems the most usable feedback comes out of anonymous feedback forms where people don't have to worry about such things. So in your case, I think the forum could work.
Do I understand it correctly that you're teaching a class over several days? That's also something that can work in your favour since you can build up trust over time. There will always be a few people who are not afraid to ask questions and if the others see you interact with those, they will be more encouraged to ask themselves. That, however, takes time which you usually don't have in a normal presentation or a one-off workshop.
I remember a presentation I attended some years ago. It was by Andrei Alexandrescu, about some peculiarities of C++. Stuff that would have went over most of the audience's heads. But it soon became clear that it was more important to Andrei that people understood than that he was getting through his prepared content. He asked the audience questions and when he was met with silence, he went back and explained it again in a different way and then asked the question again. Soon, people were not afraid to shout out answers, even if they turned out to be wrong. He would patiently go back and explain things again. I've never seen something like this in a "regular" presentation (before or since) - it was really impressive.
Of course, that was with an audience that went there on their own free will. Not quite the same situation you're in with your students. Still, I think you could try and establish such an open atmosphere, at least for the in-class sessions.
Doing that online is even harder, of course. Will the classes be live or will the students work through them on their own pace?
Just throwing out ideas here, but I think the forum would again be a good starting point. Or maybe try some social media interaction? A Facebook page, for example, or Twitter. I followed Jennifer Widom's 2011 Stanford DB Class online for a while (didn't finish it due to time constraints), and they did a few things on Twitter. She also did what she called "screenside chats" where she answered student questions in a video
podcast.
Hope this gives you some ideas.