Unfortunately, JPEG doesn't compress by line but by 8 x 8 pixel blocks, and the output from that stage is then also compressed, at least as I understand it, so to get your output you're going to have to decompress everything anyway.

Imager doesn't provide any access to the file at this level, you might be able to convince libjpeg to give you some information about each block before it converts it to pixels, but it's not something likely to ever be supported by Imager.