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.