Right now I don't think there's anything of that sort. And I'm not sure if there ever will (or should) be one in the future.
The more Mastodon-y parts of fedi heavily look down on this sort of global search, as microblogging of that sort is generally a lot more personal compared to the community oriented focus of Lemmy/kbin. And it's going to be pretty difficult to handle that in a way that both parts of the fediverse will be happy about, without giving abuse tools to people in the more "channy" corners of fedi.