Hmmm. I just read this and wanted to say, I have not answer, I am pretty new to this as well, but, your idea is great one, and valid. I run an online forum community and back in the early days when I started our forum I enabled "image by url" insertion into topics and posts. Unfortunately over time those images either become invalid or the users move them or delete them off their image hosting site, in turn leaving our forum topics with ugly empty holes with an ugly url where the image use to be. If I could do it all over again I would have never allowed it.
That being said, at this point about the only thing you can do is what you are doing, rules, and blocking the image hosting site urls in your domain blocker list. Beyond that I am not sure. Hopefully someone with more knowledge will chime in here with a solid "yes" or "no" it can or cannot be done.
Thanks for bringing up a very important issue. It may also have to go on the Plugin Wishlist at
https://forum.dokuwiki.org/forum/19