Recently I was working on a game compatible with CocoonJS with DOM limited to the canvas element - here is my approach:
Using fillText with a font that has not been loaded yet will execute properly but with no visual feedback - so the canvas plane will stay intact - all you have to do is periodically check the canvas for any changes (for example looping through getImageData searching for any non transparent pixel) that will happen when the font loads properly.
I have explained this technique a little bit more in my recent article http://rezoner.net/preloading-font-face-using-canvas,686