I tried doing this:
http://jsfiddle.net/path411/sRkWq/
It appears as though Chrome (37-dev) will not receive any data from behind the canvas, while IE11 and FF(29) will allow me to see pixels behind the canvas, but only see the unvisited color.
I don't believe there is another method to receive the color of a pixel from a canvas except through getImageData(), but I could be wrong.