Seems they flew three U-2 missions from Charbatia to Lop Nor and back, circa 1964. From memory, with ROCAF pilots.
The ROCAF's Squadron 35 (The Black Cat Squadron) used U-2s for mainland China overflights from January 1962 to March 1968 (after that only missions outside the borders of PRC were allowed).
The first idea for a flight to Lop Nor came in middle 1964, using a taiwanese U-2 but with an american pilot (the flights from Taiwan were strictly controlled by USA, and only authorized and flown by ROCAF), but at the end the mission was never made.
Only on 7 May 1967 the first mission (code C167C, the 93th overflight of PRC) to Lop Nor was made from Takhli (Tahiland), using a taiwanese U-2 on a very special mission: dropping two sensors to record the seismic activity of the nuclear explosions (Project Tabasco). The sensors would extend an antenna to send signal to a USA SIGINT station at Taiwan, but the data were never received. The analysts presumed the signals were not to strong enough to be received so far, so they decided to make another mission to Lop Nor using an U-2 with data receiver onboard, developed in short time and using an long wire as an antenna, spun from the Q-bay. The antenna would be recovered onboard, but in case of a malfunction of the reel a cutting device would cut the wire.
The mission (code C287C) was flown on the night 30-31 August (btw, it was the 100th overflight of mainland China performed by the 35th Squadron); it was a frightening mission, with a couple of SA-2 exploding near the U-2, but the pilot continued the flight, though the receiver did not record anything. During that period the first Chinese thermonuclear bomb was tested at Lop Nor (June 17), so a lot of interesting data would be recorded...
More details on Chris Pocock "50 years of the U-2", Schiffer, 2005 (and on an article written by myself in 2017 about the Taiwanese U-2s for an Italian magazine
).
You can find below the redacted mission resume of the second flight, taken from the CIA library.
