Commentary: Interesting experiment. Being a professor myself, I have come to look at the situation this way: it's become a commonality that students need to feel engaged in order to pay attention. But they need to be engaged in a certain way--one where they actively listen but also pause to absorb and reflect, perhaps even prelinguistically, upon what they've heard. (Think about how we listen to a symphony: we don't become a music critic after the first 10 measures; we take what we've heard and let it *be* there--in our RAM, if you will--and that makes listening to the next 10 measures different and cumulative.)
When students are invited to tweet/comment at a sentence by sentence pace, they are engaged, but in a way that obviates the kinds of pauses above. Effectively, they are actively distracted. There's a chemical basis for this: http://www.psychologytoday.com/blog/brain-wise/201209/why-were-all-addicted-texts-twitter-and-google One of the things I take myself as "teaching" students is a different way of paying attention than is trained by our device-centric culture. The "longform" approach to attention. Also called "thinking"!
I do see there as being ways to use Twitter to enhance the lecture, something perhaps even a bit less active than what you originally intended. Perhaps telling students they could use Twitter to compose 3 tweets during the lecture, a selection of which would be presented during Q and A, later. That way, they'd be careful about how often they broke attention without feeling "shut down". It would be a kind of lottery.
Anyway, since I've never tried this, take it for what it's worth. I appreciate Chris Long's sharing his experience and for taking risks.