There are 2 options;
For JBOD(with CSE-PTJBOD-CB2) the PMBus is connected to CB2 which has an i2c cable(CBL-0102L) hooked up to the BPN. If it is the case, then the expander could get the power supply status. You can see this in your controller management.
On the other hand, if you connect the PMBus to Motherboard, then it should get the power supply status via SuperDoctor III (i.e. via motherboard). BPN will not have such information. In this case the controller management will show Power state N/A or missing.