Could you just use a tag for this? My shallow thought is that if you
tagged headlines, over time you could use different tags for different
content type whereas if you use a new custom type, you would need to
repeat the definition process (whatever that might be) every time you
discovered a new required type. This would also enable you to benefit
from some of the other nice attributes of tags, such as inheritance.
Googling a bit after reading your answer, I fond #+FILETAGS: which I hadn't noticed before. Since all my lectures are in the same file, I guess I could just add a single line to that file and, as you say, eventually I'd have an extensible and potentially more sustainable system.
Thanks to you and everyone else! sorry I too ka while to get back, forgot I wouldn't have Internet while I was away.