I haven’t seen that, myself. That formatting sounds like it would interrupt the reading experience, forcing the editor or agent to stop reading the main text, drop down to the bottom of the page, and then go back up and find their place within the main text again. Such interruption is the same reason many people dislike footnotes in a published book. Some readers simply skip the footnotes altogether, or wait until they get to the bottom of the page to glance at them. I recommending sticking with industry standard, which is to set the illustration note within brackets alongside the pertinent line of text. Why risk distracting an editor with a formatting detail? Save your risk-taking for revolutionary story content or narrative styles. That kind of “different” wins you fans.