That's an interesting angle, thank you for that idea!

I'm going to look into it right now...

So... thank you very much Markus for your steer!

Here is what I came up with for now, in the ContentService.Saving event I used this...

        var blogArticleEntitiesBeingSaved = e.SavedEntities.Where(x => x.ContentType.Alias.Equals("blogArticle")).ToList();
        foreach (var blogArticleEntity in blogArticleEntitiesBeingSaved)
        {
            // Get the release schedule
            var releaseSchedule = blogArticleEntity.ContentSchedule.GetSchedule(ContentScheduleAction.Release).FirstOrDefault();

            // Get the publish date (this will only be set if the article has previously had a release schedule)
            var publishDate = blogArticleEntity.GetValue<DateTime>("publishDate");

            // If there is a future release schedule date and the publish date hasn't previously been set
            if (releaseSchedule != null && releaseSchedule.Date > DateTime.Now && publishDate == DateTime.MinValue)
            {
                // Do something you only want to do the first time it's been scheduled

                // Save the publish date so we know it's already been scheduled
                blogArticleEntity.SetValue("publishDate", releaseSchedule.Date);
            }
        }

It seems to work well for my needs, the publishDate field feels like a bit of a workaround but all subsequent saves of the item after scheduling would trigger too so I needed a way to only do it once (on the first time it's been scheduled).

There is a message in the UI "A schedule has been updated" that is shown when the content is first scheduled for publish and isn't shown on subsequent saves so presumably there is something schedule event related that could be hooked into but I couldn't find it.