See
CHK-4 and
http://confluence.atlassian.com/display/DOC/Extractor+Plugins
The metadata plugin already has MetadataExtractor, that reads from the actual Content properties... so in theory that should do it...
question would be how to trigger the extractor...
As per the note in the extractor plugins,
"Each time content is created or updated in Confluence, it is passed through a chain of extractors that assemble the fields and data that will be added to the search index for that content. By writing your own extractor you can add information to the index."
The thing is that the checklist goes and modifies the Content properties without modifying the page... gotta be creative on this one 