I am afraid that is completely off the table, if we did something like this we would allow third parties to track usage on a forum by injecting a tracking gif. The download remote image thing is a part security feature.
Instead I think we need a “smarter” system that works similar to the way @tgxworld built our image remappers a few years ago, one that works backwards from HTML and ensures stability of the change by re-cooking. Very complex change sadly.