but there is presently no analogous operator on Google's search for a distributed system that spans many domains.
Because that's just a basic search. A search engine searches across multiple domains by default. If you're specifically looking for only results from ActivityPub enabled services, that's pretty much an impossibility since there's no way to know (from a web crawl) if a page is served by a server that supports ActivityPub. Another problem is that a lot of fediverse instances purposefully block search engine crawlers because they don't want to appear in search results.
ForestOrca