I have used http://fabricjs.com/ in another project (not related to printing). FabricJS is used/developed by printio.ru . Its a good open source alternative.
Fabric.js is canvas-based, so it's a slightly different beast (although between being used in t-shirt designer tools, and having svg<->canvas conversion, the ultimate result is the same).
I'm using fabricjs for annotations as well and it's a great library. The biggest downfall of using canvas as a renderer as opposed to svg is that it doesn't look great if you provide zoom controls. Lovely crisp SVG paths rendered as pixels on a canvas :/