The regime is so oppressive I wonder if even a handful of Witnesses exist. I believe Albania has the same situation. It was interesting b/c I went to a lecture in law school by Amnesty International workers when they won the Nobel Peace Prize. Moot court was taking place so there was a small turn out of mostly faculty. It was an intimate gathering. They revealed they know almost everything that happens in Russian and China under Soviet type governments. North Korea and Albania were completely closed. It usually helps political prisoners to live in a city. Nothing helped those in No. Korea and Albania. Also, the Soviet Union and China could be pressured. Any letters to No. Korea and Albania would only make it worse for the prisoners.
They can't announce publicly if there were Witnesses. Perhaps knocking on doors is suspended. Maoist China had a Roman Catholic structure hidden in the country. To be fair, has anyone not watched the videos of No. Koreas in abject misery b.c of the death of their leader not seen some comparison with how happy Witnesses say they are. No. Korea's fervor was helped by a prison sentence of ten years for not wailing and prostrating oneself on the ground.
It seems the WT does not have a universal policy towards repression. If there are any interested people, I hope they know how to hide it very well.