I'm not nearby my MP installation, but if you push the "blue" button using the MCE remote when playing a video or DVD, the osd text for "aspect ratio" /"zoom mode" would produce a black text without a frame or other typical osd graphics, in the upper right corner. But movies are often dark and the text is therefore more or less invisible... the same goes for changing subtitle using the "yellow" button with my MCE remote. You can of course use keyboard shortcuts too... 
Germany