The idea would be to check if a callback for a specific object type exists. If it does not exist, look to the supertype for the implementation. This could be used for the datasource or url.
This might be useful for xowiki, since you could create a datasource implementation that used introspection to get the attributes of a subtype of xowiki::Page and generate the search datasource from that. Since the method would be the same you would not have to re-implement it for every substype of xowiki::Page.
Right now content_revision has a generic implementation that is almost always wrong, so we would need to deal with that. Generally generic content_revisions are not interesting objects, if you wanted them to be indexed for search you'd create a subtype.
