They can use a solution which already integrates the search. Forums and CMSes are a good target for that. Then you can say "I'd like my search to look at widgetlovers.com too" - and you get their sitemap + featured external links, because they run FooPress that supports it.
Kind of the same as sitemap we already produce for Google.
I imagine a binary, with a simple Admin UI allowing you to crawl some domains recursively would be enough to index your own website, and then have those results shared.
Where I could see this being really useful, is let someone who knows everything about pokemon provide the index for searching pokemon information. Then when they federate, provide a taxonomy saying "for queries that have these words, call me". Suddenly you have a very high value search source for pokemon.
Throw in some zero click info information boxes and you have added a lot of value.