According to the SMF forums, there is no way of embedding WebM files without recoding (according to the SMF manager). As a not particularly technical individual, I make the assumption that mods/addons would therefore be unable to support the files (there aren't any mods regarding them on the SMF forums, so I assume this to be the case).
I suppose it's possible that it could be enabled in updates to SMF itself, but I have no idea if it's on their list of things to do in future updates.
Hopefully that provides at least a little useful information for people.