test streaming youtube with horizontal format:

needs long captions

if the embed code is longer than 255 characters, it will also be necessary to edit the database and change ndxz_media.media_caption from tinytext to longtext

create a transparent gif (greyscale) with the width of your video and 1 px height. You can also do it with a jpg in the same colour as your background

upload the image: you may see warnings, but the image will upload anyway
(dynamic thumbnail creation doesn't accept 1 px height? then it won't work with formats that use thumbnails)

paste the embed code into the caption area (not the title)

You can have a title, but as the title is positioned above the caption it will appear dodgy mixing still images and video in the same page. Would require some editing of the css (see Horizontal format) and some editing of the horizontal plugin

It's a hack, but it's a good hack:
thanks to sannemaloe who provided the hack in this thread