You shouldn't need to create triggers because the comments are stored in the content_repository.
The datasource service contract implementation returns the actualy content to be indexed to the indexer. So for general comments, it would be the comment text.
The implementation_alias is the tcl proc that actually implements the operation for the service contract.
Using the search package with OpenFTS, there isn't a solution yet to only show the link to a parent item. That can easily be seen by searching here at openacs.org. Each reply to a message is indexed, and a long thread can show many listings to the same thread for a search.
