I want to load an SVG image, do some manipulations to its .contentDocument, and then draw it to a canvas.
.contentDocument
A good example for drawing an SVG to a canvas
Convert the svg's xml content into string data, then make a data uri out of it. You should be able to load that as an image. Something like this:
new Image("data:image/svg+xml," + svgdata);
Now you should be able to draw it to the canvas.