I haven't looked at the API yet, but the demo is great on my phone. Instead of just justified blocks, can you have it do a "justified" (even amount of spacing) from an irregularly shaped right edge (in my case, an image on the right side of a design template I made)?
That's not something justif supports directly right now. It currently targets rectangular blocks, not arbitrary edge shapes. But the underlying line-breaking approach could accommodate per-line widths, so it would be plausible to add I think.
Feel free to make a GitHub issue with details and I can take a look at supporting it. For now the workaround would be falling back to native justification by excluding the paragraph from justif, and using css float/shape-outside to get the desired edge.