I noticed yesterday an "External Networks" link inside Yammer. I don't know how long it's been there, but it's similar to what you are describing. As described

I noticed yesterday an "External Networks" link inside Yammer. I don't know how long it's been there, but it's similar to what you are describing. As described on Yammer: "External Networks are separate Yammer networks used to communicate with external groups, like customers, partners, contractors, and fans of [company]." The drawback is that an existing Yammer domain would "own" or have control of that network. I know it's probably not exactly what you were looking for, but it might be one way to get to use Yammer istelf.