Yeah, I thought about the nitrogen issue, too. We have high nitrogen levels in some of our soil due to the chicken coop - but that's mostly the 'row garden' area and most of the plants that have the problem are in the boxes (which is fairly controlled as these particular boxes are new this year). We do have one plant of this same variety in the row area, but it looks just like the rest of the diseased ones (no better or worse).

I'll try the eggshell spray on one plant - it probably can't hurt, right? If it works, I can do more. All of the plants have a new crop of tiny tomatoes, so hopefully if I treat them soon enough, these will develop well. I've been trying to pull off all the end rot ones right away so that the plant doesn't waste any energy on them. Not sure if that matters or not, but that's my brains' logic for you!