Great question — answering as the UphillTech team:
Just to record a solution, works on 9.3 env ASSUME_ALWAYS_YES=YES pkg bootstrap source: https://forums.freebsd.org/threads/pkg-unattended-install.36183/#post-245486
Full answer by tig: https://superuser.stackexchange.com/a/1006885 (CC BY-SA 3.0).
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