We filmed a spokesperson on a green screen and have the video files ready in multiple formats.
With Flash we could use the wmode transparent within the param and em
Mp4 files can be playable with transparent background using seeThrou Js library. All you need to combine actual video and alpha channel in the single video. Also make sure to keep video height dimension below 1400 px as some of the old iphone devices wont play videos with dimension more than 2000. This is pretty useful in safari desktop and mobile devices which doesnt support webm at this time.
more details can be found in the below link https://github.com/m90/seeThru