> Yeah, one of the things which I always wished for when doing page composition was a way to visualize which paragraphs could be set a line or two longer or shorter while still being set reasonably nicely.

I wrote a LaTeX package [0] that does exactly this. The default settings automatically lengthen paragraphs as necessary, but you can configure it to only tell you which paragraphs can be easily lengthened [1] without actually lengthening any of them.

[0]: https://www.ctan.org/pkg/lua-widow-control

[1]: https://tug.org/TUGboat/tb43-3/tb135chernoff-lwc.pdf