Mpg (and mkv and ogm for that matter) Attribute Support

You can see who wrote a plugin by going to Settings -> Preferences -> Plugins, selecting the plugin and then clicking About.

In the case of the Movie plugin it was written by GPSoftware, the same as Opus itself, so wherever the functionality would go it's probably them that would add it. (Although someone else could write an additional plugin to do the same thing without replacing the Movie plugin.)

You can write to GPSoftware here:

If you want to make MP4 work then see this FAQ: HOW TO: Enable/fix playback of various media formats in Opus.