Solution
It is impossible to maintain impedance during measurement because the SMU should have low impedance when applying voltage to the DUT. The high impedance observed at some points is likely due to the SMU transitioning from its power-up state, where the output disconnect switch is open, to its initiated state, where the switch is closed. In DC voltage mode, the SMU behaves as a low impedance device by definition.
The PXIe-4143 module does not support the "output connected" property, so there is no way to programatically control this switch state. The SMU can simulate a very high impedance between the HI and LO terminals if configured for "DC Current" mode with a current level of 0A, ideally in the smallest current range, depending on the desired impedance level. However, this will not simulate a constant impedance, as the SMU will attempt to simulate as close to infinite impedance as possible. Therefore, there is no workaround other than using an external resistor.
