Sorry, forgot to update here that the Managed Commons project now resides at Github: http://github.com/monoman/Managed-Commons
It includes the WebM subproject, that will allow reading/writing WebM files/streams generally and decode/play such streams in Moonlight.
Little time to work on it, so the progress is very slow, but if you want to contribute, please do: Fork it at Github and send those fantastic Pull Requests.
Also please post issues there to help guide/prioritize development, I'm trying to first be able to read the Matroska files, them I'll start the decoder and pump some video data, and last plug it into Moonlight. Writing streams/files has low priority at this point, unless someone really want to develop a video producing app, or a slideshow-to-video converter and want to contribute code and testing to that end.