But you can just... Decide how many clock cycles you'll need to finish shutdown, calculate that power usage, then solder a corresponding power source in place. There really isn't guesswork.
Assuming you only need to execute one Flash transaction (doubtful!), covering all of the variation might take upwards of 50ms.
So, yeah, you could just throw a massive cap on the power rail. Except that that conflicts with the other power, cost and size constraints implicit in the context of microcontroller systems.
If your power supply is already 1.8V, most uC will not have much room for procedures when a POR/BOR is detected.