This issue has been mentioned in pull request https://bitbucket.org/hudson/magic-lantern/pull-request/576/external-monitor-fixes-for-old-cameras/diff
When viewing MLV_PLAY on and external HDMI monitor, Top and bottom borders of the display (which should be masked in black per image size) display garbage. You can see an example of this in the following images:
When I attempt to capture a VRAM dump of this on 7D, the filepath is created, but it's just an empty folder with no dump inside.
I would like to attempt to understand the reasons for this in the following thread in "general development" section of the forum: http://www.magiclantern.fm/forum/index.php?topic=13156.new#new