I have added a search for Captioned Video on Hulu to my Captioned Video Search Experiment.
It’s based on their “Episode Lists” containing “(cc)”.
They don’t currently have a search by captioned video, but if you go to a show, and look at the episode list, it shows which ones are cc. (you have to hover to see them).
You suggest CC search as a feature to them, http://www.hulu.com/support
Sample Search: http://ccmovies.org/fsearch.php?searchterm=30+rock