I’ve found a solution-of-sorts, and was wondering if it might point to another possible hook area …
I was playing around with AdRotate plugin for other reasons and noticed they had an “After Content” setting. I wondered where it inserted content “After Content,” and it turns out it does so right where I wanted to–at the end of a blog post, before the post meta and Jetpack related posts.
So I took the content I’d had in my GP Hooks After Content and moved it to AdRotate. I’d rather have it in GP Hooks, but this will do for now until it breaks my website 🙂
I haven’t moved the “After Content” content on all my posts yet, just on single posts in /category/for-writers/calls-for-submissions/. But if you want to have a look, that’s the end I was looking for.