Isn't that what Overwolf the overlay app does i mean it doenst have youtube but you can browse in the youtube site and it is an overlay feature
not sure, i seen a few youtube mobile players, but what they do is just load each video link as u search for it as many websites repeat youtube video do, what my request is, making a area for login or register to youtube, load your playlists and automatically play them as you do in youtube, but it could be used for mobile, pc etc . its taking so much of resources of using youtube online, it creates a big cache, so by running it from pc will be easier... and less resource eating.