You're right, sadly it doesn't work with mplayer directly. I used to run
wget -O - | mplayer -
but now that also broke because of some seek issues.
It should be trivial though to change it to
wget -q -O /tmp/yt & (sleep 3; mplayer /tmp/yt)
I'll give you an example of my issue with youtube though: I get a rendering that looks like CSS is disabled and the message "This video is unavailable."
This now triggered deeper investigation. It might be related to the SSL problem you mentioned. I removed the whole .fifth folder and youtube started working again (perhaps cause the javascripts and css files come with a different certificate?).
Now with new SSL providers giving out certificates that are good for only few weeks certificate pinning sadly got less useful for the general internet

One issue I have on youtube still is that clicking the "Stream" button is kind of difficult cause there's that defunct play button from youtube blocking most of it
