Hey i have a simple idea for the cinematics. Why not put a link to the youtube videos of the cinematics so people can copy the link and alt-tab out to watch it. It breaks the immersion a bit but its better than nothing.
Or just have a separate download for the cinematics (you can play videos in sc2. Someone imported a rick roll video into a beta map, unless maybe it was just pic from the video strung together real fast) where each cinematic is a map that has to be loaded like any other.