After procuring a (Chinese low cost) high impedance insulation tester I obtained these values across the disconnected platen for different test voltages at room temperature:
250 V → 208 MΩ
500 V → 198 MΩ
1000 V → 186 MΩ
These readings were done after about 1 minute, when the value had stabilized.
The clips were attached to the connector at the end of the short cable (about 50 mm / 2 inches) of the platen.
Does this sound reasonable?