I think your suggestion that the British clothed him during his trial is the likely explanation.
Commandant Scheepers was diagnosed with obstruction of the bowel and appendicitis on 11 October 1901, he was gravely ill and could not move, and was captured the same day. His trial began, after postponements, on 27 December 1901 and he was executed on 18 January 1902. The photo shows him sitting up looking relatively healthy, so I think it probably dates to around Xmas 1901. He was able to walk to and from the ambulance by the time of the trial, though he was executed sitting in a chair. If he had been offered a selection, Scheepers may have chosen this tunic for sentimental reasons - it is reminiscent of the uniform he wore in the Staatsartillerie.
The past is not dead. In fact, it's not even past.