I would not mention this unless the "anything else" essay is prompting you for this type of experience. You could use this essay to address the mandatory prompts though (such as the 'challenge' essays if you don't have a better topic)
If this didn't make it into your 15 activities, that either means it wasn't that significant or it was your 16th most important activity (i.e not that important). Unless it was something truly impressive (in which case, it should have been in the primary application), I would not include it.
At best, being the 16th most important activity, it is unlikely to sway anyone's mind. And at worst, it could be seen as filler. You can bring it up during interviews though if asked about leadership experiences. Just my thoughts
