That location could definitely have been for a second knock sensor from the factory. The Skyline R32 (which has an almost identical ECU and knock control system to the Z32), has two knock sensors. On the R32 the sensors only listen to two cylinders each due to the layout of the inline 6 engine (i.e. R32s only get knock detection on 4 out of 6 cylinders). Our single sensor can listen to all 6 cylinders, so multiple sensors aren't necessary. It's possible that the Engineers considered a second sensor to lower the signal to noise ratio for cylinders #1 and #2 , or it may have been used to compare different locations during R&D.
One thing to keep in mind about knock control is that per-cylinder calibration based on sensor location is absolutely critical for accurate knock detection (i.e. cylinders #1 & #2 are furthest from the sensor and are therefore much 'quieter' for a given knock level; higher sensitivities are required to detect knock on those cylinders). For this reason using the second sensor location for the OEM knock system wouldn't be a good idea. To relocate the sensor you need to re-calibrate the per-cylinder knock thresholds in the ECU and that requires an engine dyno with cylinder pressure monitoring...
