>Correct, but does it exist right now and are people using it?
Yes, it exist right now, but not many people use it. Implementing it in browsers wouldn't really require more than supporting the container in the first place. VLC can already stream Matroska files just fine and technically speaking any other media player capable of playing Matroska files should be able to as well (I haven't tested this myself, but judging from the way it works there should be no problems).
>It will take more work, and requires the files be assembled for streaming
Not really. Basically it just means that your file is muxed without an index, and even if it was muxed with an index you should be able to play it just fine if it were being live streamed, while missing the index. The only extra work involved is setting up the live stream, and you have to do that with OGG as well.
In general, I'd say that the main reason why people aren't using Matroska for live streaming on the web is that web players for Matroska don't really exist. Flash doesn't support it, and there's no Java plugins or anything. Though I guess that VLC/MPlayer plugins in browsers should technically be able to handle live Matroska streams just fine. HTML5 <video> is a great opportunity to bring Matroska to the web. Just because other things have been used earlier does not mean that better (and completely free!) options couldn't be implemented now.
And to clarify what I mean by saying that Matroska is better: It's a better overall solution. OGG might be equally good if not slightly better for live streaming, but considering all user scenarios (including local file playback), Matroska wins hand down. It's features and software/hardware support simply blows OGG out of water.
Once again, live streaming is a very minimal part of video on the web, and choosing OGG over Matroska because it might be slightly better at it is simply put a stupid idea in my opinion.