We confirmed that MM player respects _alpha while playing video.

This could be implemented easily by the renderers but the current design does not support color transformations:

void drawVideoFrame(image::image_base* baseframe, const matrix* mat, const rect* bounds)

...should become:

void drawVideoFrame(image::image_base* baseframe, const matrix* mat, const rect* bounds, const cxform& cx)

And, of course, the video instance must deal with _alpha and such (haven't checked if these properties exist already).