Hand saw (22in or 24in). A multitool has insufficient capacity, plus with lath and plaster you are likely to find that the vibration wil break the snots and part of the ceiling will just fall off.
Do keep up, Andy! Robin mentioned it in post #25, I mentioned "old hand saw" in post #29 and there have been at least 4 mentions since then (panel saw, hand saw, etc)...
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