Mark, thanks for finding time to share your ideas, especially in such a busy travelling time.
Fortunately or not (?), this type of sessions are becoming a reality. We had a similar situation during the EUMETSAT Conference. It was a challenge consisting of remote, on-site presenters and audience. We used web-conferencing and live streaming. I was happy to hear that it is an experiment, as I was one of people responsible for it. I felt a silent permission to make a small explosion. And we did have one :) Now I know even better how much experience and practice is valuable.
Technology is an important factor, in this case adding even more stress and equipment to handle. But there are technical people who can usually help. Testing and backups usually work. Except those few times they don’t... As there are more gears in this machine, surely there are more possibilities that something will break.
Anyway I would be more cautious about the learning aspect, audience, different pace (or delay), interaction and communication channels, accommodating presenter's need for feedback from two audiences, on-site and remote moderators, than technology itself. For sure one-way presentations are easier to handle but they have a limited use too.
Two tiny almost real examples combining interaction with on-site and remote attendees. With a little more preparation they would have been excellent.
REMOTE PRESENTER. One is a question by a remote presenter to both audiences, where people have to raise the hand (virtually and physically) in order to answer YES. A moderator is supposed to count on-site YESes and forward message to the remote presenter. But with a good webcam (or camcorder) the presenter would be able to count it himself.
ON-SITE PRESENTER. Or another presenter distributed handouts/images on-site, and followed up with a simple exercise. With some planning ahead the same could be done simultaneously online? Digital copy of the image followed up with a poll.
I am not saying it is easy, or that it always work, or that we have a recipe already, but it is an area worth exploring as you said.
I like to use the word control mostly when referring to the CTRL key, perhaps that is why I am not an elementary school teacher :)