You know, that's a really good question - I don't think we considered that use case.  Probably because we were expecting the rendering process itself to not take a really noticeable amount of time.    Other than rendering a new image component, or a complex canvas, or a really complex table, I can't imagine it taking all that long - what are you rendering?

(If it's just a single image tag that's rendering, you could always do a onload attribute on that element...)

Though I haven't tested it, I think that a rendered event isn't going to help you anyway if it's an image, since the image itself will be loaded out of band (I think) after the DOM is updated, and the rendered event will have long before fired off...  more or less mimicking what you're already seeing.