I did this on the Vocaloid wiki. I think I figured out how to link Niconico thumbnails.
Example, MikuMiku ni Shite Ageru in Sandbox
I edited the MediaWiki Whitelist to include smilevideo in general. On a this site I used source code to see how the thumbnails were displaying, and saw the &dummy.jpg in the coding. I am puzzled, For example (thumbnail at bottom is niconico), how other sites are getting the thumbnail to appear without the &dummy. It may have to do with Wikia's default script, something the regular users can't access.
I have no idea if it functions similar to YouTube thumbnails, as being able to tell when a video has been removed and leaving some kind of default 'no video' image. Because I tested it with the original JBF, and since the video is likely hidden and not actually removed from the site, the thumbnail is showing up.