I actually looked into that, but it's complicated--while 30-second clips seem to be "customary" at some level, that custom doesn't seem to be endorsed by actual court decisions or statutory provisions (and rights organizations ASCAP and BMI take the position that such clips are not fair use). So I'd be taking a risk, including the risk that the publishers might yank my license to distribute. Alternatively, I could go to them and seek permission. I've been reluctant to do so up to now because I didn't want to do anything to complicate getting the licenses--it took long enough as it was, and Tim's company in particular seems very leery of stuff involving the internet. Once I've submitted my first quarterly statement, I may go back to them and see if I can get permission to post a short excerpt.
This rights stuff has proved to be a pain, but I'm determined to do it right.