Not a bad idea, but not a great one either. Milorganite takes a few weeks for the nitrogen to become available to your grass since it is organic and needs to be converted by your soil microbes. Summer is not a good time to force your grass to do anything. Milorganite is pretty gentle so it's not going to force growth like synthetic fertilizer, so you can apply it if you want, but don't expect you grass to look like Yankee stadium.