Please note that technically, flash hook and polarity reversal are two different things. Flash hook is brief a opening of the circuit by a phone to the CO, so in fact an FXO card can generate, but not detect flash hook.
Regarding polarity reversal, that FXO card can detect but no generate, it may used by IOS for answer supervision. I do not know of a method to have them generate SIP INFO hookflash, becase as mentioned above, it is a different signalling event.
One could try if a TCL/IVR script is passed the event of polarity reversal, but I'm not convinced it is.