How to build custom level editor components

In Unity, I built Editor only programs to allow the artist to place items in the level with my program doing all the dirty work of building the underlying objects/prefabs and meta data. In UE4, it sounds like I would have to hack the editor code to do this which would go beyond the practical level of effort versus benefit. I am wondering if there might be an API to run as an editor function. Where would I find it, if so?

To try to describe what I would be producing, the program would allow selection and placement of previously developed objects or blueprints and let the artist drop them visually, then my program builds the meta data to reproduce that at run time (which then gets read and acted upon by the actual game code). It’s rather like the old sparkedit you’d use to mod Halo I (loose analogy, but similar). Such a program can be quite beneficial in rapidly developing follow-on levels and games for the same series. I’m not seeing how to do that in UE4.

You can write your own Editor Plugins in Unreal. Some links about plugins:

It is possible to do almost anything you can imagine, but it will require serious digging into UE4 Editor source code.

That sounds like what I am looking for. Thank you.