As I see it you want to filter content items, so the case is different than indexing pages. Parvan suggests that firstly you should index all items and then in searchResults you should show the filtered items.
Here is a sample code of this implementation.
1.Create a custom SearchResults control that will show our filtered items. Override CreateChildeControls adding our custom filter.
2. Create your custom SearchToolboxitem.
3.Register the control in the web.config file.
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