I can see one way to do it, but it's kinda dumb.
The GIFV URL is:
http://i.imgur.com/zvATqgs.gifv
But the HTML references
http://i.imgur.com/zvATqgs.mp4 and
http://i.imgur.com/zvATqgs.webm
So I could do it but only if the tag looks like this:
[gifv]http://i.imgur.com/zvATqgs[/gifv]
Which is dumb.
I guess I could also do
[imgurgifv]zvATqgs[/imgurgifv] as it's an Imgur only thing at this point and not a standard.