There is, at this time, no interface to do this. Our scripting engine (and by extension WCS' as well) is currently not being run during movie playback (In-Engine cutscenes that are rendered in real time are a different matter, of course. Implementing subtitles there is trivial).
Also note that even if we decide to implement such functionality, it wouldn't help WCS, as they have an engine that is vastly different from ours by now (They forked the engine years ago, and we've made quite a lot of changes since then that they haven't copied, and they've made quite a lot of changes to fit their game better that we have no access to).