Probably depends on the manga itself. Action manga probably don't compress as well as more dialogue-heavy works.

I would, at a first blush, disagree with that characterization? Dialogue equals more fine-grained strokes and more individual, independent “zones” to encode.

I wonder if the text would be consistent enough for JXL's "patches" feature to work well.

Now I'm really curious. If anyone has more time than me they could do some sample compressions and I'd be interested in the results.