Reversing P-channel mosfet for polarity protection component selection
FETs ideally don't care about the direction of the current. You simply have to invert your conventions and switch Source and Drain in your equations. Everything works exactly the same way.

The only reason that power FETs do care is because there is a Source-Body connection put in place during device fabrication. This creates a parasitic diode that conducts when the voltage is reversed. However, if your drain-source voltage is below the diode voltage, the device will still behave as a FET.

If you have access to the fourth, body, terminal. You can bias it externally so that the device always operates as a FET.