Google has a new way for sites that use Google Custom Search to add pages to the custom search engine's index.
"Our users have been telling us that while they love the concept of on-demand indexing, manually entering URLs or keeping a Sitemap up to date is too cumbersome," says Google software engineer Liqian Luo.
If the site is verified with Webmaster Tools, you can provide Google with a URL for your page that links to your site's latest content, and Google will "periodically" visit it to discover the new content.
Luo writes in a post on Google's Custom Search Blog at googlecustomsearch.blogspot.com, "For example, as the owners of this blog, we can provide the URL of the blog’s landing page, googlecustomsearch.blogspot.com, to our CSE. It will then periodically visit that URL and add any new links to content on this blog to our CSE’s index. Since this URL automatically links to all new content on this blog, our CSE’s index will always automatically get updated."
The URL can be provided via the "URLs linked from a page" feature in the indexing page on your Custom Search Engine control panel.
As Luo points out, it only works for discovery of new content. Not refreshed content that has already been indexed.