Sarah - At Benetech (501c3 nonprofit) we've been working with the developers on the Learning Registry project to enable searches to be filtered by accessibility properties (e.g. captioned videos or blogs with described images). We would also love to have developers who are interested in accessibility to contribute to this effort. You can check out the LR-Wordpress-Plugin accessibility branch at:
https://github.com/adlnet/LR-Wordpress-Plugin/tree/accessibility

and a alpha version working version at:
http://www.a11ymetadata.org/implementations/learning-registry-search-demo/

We also have been advocating the use of Schema.org accessibility properties with Wordpress developers and had the YouTube Lyte plugin has added support:

We encourage other developers to take advantage of these features to enable discovery of accessible resources on the web:
http://blog.futtta.be/2014/02/03/wp-youtube-lyte-1-4-0-support-for-accessibilityfeature/

Sample search:
http://www.google.com/cse?cx=001043429226464649088:s0bmhsefbzq&q=more:p:videoobject-accessibilityfeature:captions

If you have questions, you can reach me at gerardoc benetech .org.