Why not to use some console mplayer frontends?
For example http://mplay.sourceforge.net/ is not bad.
You can find other here http://www.mplayerhq.hu/design7/projects.html

It's an interesting idea to add a video feature. But just starting external applications is not the way to go IMHO.
There are a few obstacles that would need careful handling:
1. MOC plays music via it's server part:
- Consequently, videos would also need to be played from the server thread.
- How can we attach an X-Window (or libaa/libcaca for that matter) to have one or multiple views on that video.
- How to do this from the client ? (separate video-client ?)
2. MOC does not know much about X (except XTerm afaik):
- How can we avoid having to depend on X.
3. MOC is centered on processing audio:
- Video is quite different from many aspects (the sound-hardware is responsible for keeping the framerate - how to ensure this with video ?...)

...I guess there are even more things.

A quick and dirty solution would be to write a plugin pretending to do audio but really starting mplayer and just reporting the position of the stream. Still one problem: You are at the server side and have to figure out where the hell the user expects to see that window... :-)

If you want to follow the video path further, you should try to come up with some use-cases (what you expect to be doing with the feature). From these, the real problems should be identifiable.

One thing that will slow things down is the lack of time/interest for coding addons. I personally find this interesting for purely academic reasons (can it be done ?) but see only a low value for my daily routine.