I think this is far too risky.

This doesn't help you if you want to save bandwidth, it worsens it.

It doesn't help you if you custom-crop images depending on the viewport size, because if you go that far to art direct, then you're not going to like the result of automated and unsupervised seam carving.

Just publish 3 sizes, maybe crop the smallest one if the focus area is too small. Done.