Just add a million canvas elements to your page all with the same data-processing-sources attribute, so they all load the same file. Processing.js will build as many sketches as you ask for, it doesn't care that the sketch files are the same for each one =)
(Note that what you described, one sketch instance rendering onto multiple canvases, giving each a different image, is not how sketches work. A sketch is tied to a canvas as its drawing surface. However, you can make a million "slave" sketches whose sole responsibility is to draw images when so instructed from JavaScript, and making the master sketch tell JavaScript to tell slave sketches to draw. Note that this is very, very silly. Just make JavaScript set the image, you don't need Processing if you're just showing images really)