The problem is that a higher than normal output power (no attenuation) at the AD9361 output could cause PA overload conditions.
What if I switch the port outputs from A to B (in our system the A port is used and the B port is not used) just before the Tx cal and revert to port A after cal is completed. I assume this is going to work since the Tx I/Q imbalance is not significantly impacted by the output amplifier port.