It does seem tricky to implement. My first thought would be a tick box on a Sprite's properties to change whether it does what it currently does with load url, or use new method of unique images (thus ALL instances either have the old way or the new way), but I assume that's not ideal as it's a "global" setting, not per instance.
Thankfully we do have dynamic animations to fill the gap, although few bits needed to fully fill the gap, such as setting origin point or image points at runtime and such - iirc if you had origin point at mid, and then load url, it keeps origin at mid, which could be desirable.
Definitely something I'd love to see, very interested in anything allowing "modding" support, such as loading external images.