As timber expands and contracts at a different rate to plaster, any plaster finish would start to crack over time and fall off. If you really don't want to remove the T&G, then I would recomend Plaster Boarding it. This would still move but at a smaller rate as the boards are screwed to the T&G and not Stuck/adhered.
personally I Would remove the T&G, the extra work would give better results in the long run.
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