Is there anything else I need to do to move this forward? The patch works on all terminals I and others have tried (11 in total; some with multiple versions and/or configurations). The code no longer special-cases ZLE_RPROMPT_INDENT=0, which is nice. There is branch for ZLE_RPROMPT_INDENT=0 but it's just an optimization to avoid an unnecessary zputc(&zr_cr) call when it's known to have no effect. So I think the logic is simpler now.
Roman.