As I said.... you can do that already with LUA. I think in this case you are making assumptions about what the limits of LUA are. Lua can be used to to as much (or as little) as you want in your game. Including running AI, etc. As a matter of fact, commercial games already use LUA this way.
If you look around the forums there are threads that discuss this in depth, including one where someone set up LUA to be able to call any DBP command from the LUA script.
That sounds like a little more than just a "cinematics" scripting language to me.
Design documents?!? What design documents??? I thought we were just going to wing it!!!