lectures are very much "works". And you're right, tags and annotations are currently the best way to identify them.
I think a work type would be the best way to capture this. That type doesn't currently exist, but is on a long list of potential work types that we brainstormed into existence at one point.
I don't know if that list (or anything on it) will ever become official, but maybe it will at least inspire the folksonomists out there.
edit: While typing this, I remembered why I'm not very excited about that list. It's because work types as they are currently implemented are "choose only one".