Since first posting this question I have uncovered a lot of new information. The most important comes from the interrogation of a Luftwaffe Prisoner of War by the British in April 1944, cited in an official intelligence report in October 1944.

Flights went from Bulgaria to Ningxia and were said to be for urgent items. Voyages by U-boat took 3 months and half the U-boats dispatched were sunk en-route. Bulgaria was used because it was considered a neutral country and acceptable to Japan for commercial airline flights over the Soviet Union.

In February 1944 when KG200 took over all long range special missions, Hitler tasked the head of his own VIP flight Hauptman Braun to form a Japan Kommando flying an air bridge to Japan. This unit was also known as 14/TGS.4 (or 14 Transportgeschwader 4).

A Ju-290 A-5 aircraft was requisitioned and modified at the DLH maintenance facility on Rusnye aerodrome at Prague. Not at the nearby Junkers factory in Prague. Deutsche Luft Hansa pilot Flugkapitan Rudolf Mayr was placed in charge of the Manchurian flight operation. Aircraft were stripped of armaments, military markings and were given civil DLH markings.

A War Ministry report (AIR 40/203) detailed in mid October 1944, a POW gave information under interrogationthat since the beginning of 1944, there had been "regular air travel between Gernmany and Japan established for the transport of high officials flown by old experienced Hansa pilots."

Deutsch Luft Hansa pilots thought to have flown these missions to Ningxia included Flugkapitan Nebel, Flugkapitan Mattias (died April 1946 in Soviet captivity), Flugkapitan Erich Warsitz, Flugkapitan Quenzler (Dornier Test Pilot), and test pilot Karl Patin.

Trial flights began with Ju-290A-5 werke # J900170 Luftwaffe code KR+LA. This aircraft also had KG200 codes 9V+DH. It was destroyed by air raids at Reichlin in 1945. It’s fuel capacity was increased and for long range operations, MTOW was increased from the Ju-290’s standard 41.3 tons to 45 tons.

In March 1944 three other Ju290 aircraft were transferred to LTS.290, stripped of all weaponry and civilianised for Deutsch Luft Hansa service on flights to China. Modification included fitting fuel tanks for 23,800 litres.

These aircraft were Ju-290A-9 werke # J900183, former Luftwaffe code KR+LN. From February 1944 this aircraft became T9+VK. It was attacked on the ground at Finsterwalde in April 1944 and scrapped at Travenmunde in September 1944.

Also Ju-290A-9 werke # J900182, former Luftwaffe code KR+LN. From February 1944 this aircraft became T9+UK. This aircraft was lost whilst on the ground refueling to strafing fire by four Soviet flown Hurricanes near the village of Utta, near Astrakhan.

Ju-290 A7 werke # J900185, former Luftwaffe code KR+LP was the third conversion to become T9+WK. Later in it's career it was attacked over the southern eastern front in May 1944 and returned from the mission beyond all hope of repair.*

On 2 September 1944 Ju-290A-3 werke # J900163 Luftwaffe code PI+PQ was ordered to be converted for a mission to China to carry VIP Ulrich Kessler, but work on the aircraft was interrupted by general anti Nazi uprisings in Bulgaria where the aircraft was to fly from. The aircraft was eventually blown up in May 1945 to prevent it's capture.