Sharepoint - How can I use SharePoint Server 2013 to search for Yammer content?

There aren't any solutions (at the moment) for federated search with Yammer. As you state, you can use the REST API to add an additional search vertical to a search results page. This requires that you custom-code that solution though.

This is something that the Yammer product team is working on though. It will be coming to Office 365 soon.