Not sure that would work on account of the amount of different sources of videos. I know that with other content filters you can get more specific about how you want this but not sure if Meraki is there yet with what gets blocked specifically on a webpage.
Couple things to try would be do a packet capture while trying to watch a facebook video, see if you can find a url that you could blacklist. The other would be to try bandwidth limiting all facebook access to something that can't handle video (50kbps), but can handle text. It won't block it, but the videos probably will never load.