I don't think this is going to work out. With the way progressive loading works in JXL, you don't really hit "good looking" points aside from at DC resolution.
Just being able to get 1/2, 1/4 or 1/8th of the image resolution would already cover most of the srcset use case and the results are good enough for many existing image decoders to already use this optimization for the decoding if not the network part.
