You could have just messaged them with their username. If you clicked on the username it would bring up their profile. It was easy to get to the original poster of a photo. It was easy to get to them and easy to get to their tree.
If I understand you correctly you believe you will be notified when a comment is made on photos. Ancestry has had some trouble notifying people in the past and even through member connect there is some limitation. I wouldn't rely on that. I think you may still have to check your photos regularly for comments. Part of the concern is that this looks like a lot of work for ancestry and will they be able to keep up. Indexing is ancestry's responsibility and that has always been very slow. Updating the last time someone logged in is ancestry's responsibility and that has rarely been accurate. I think they get a bit overwhelmed at times.
Perhaps they have some new system where we can be confident of getting notifications?
They don't seem to have fully implemented this yet since they are still only showing a select number of trees attached to the photos.