Do you mean that on the receipt of a external signal, that you want to save data before the mcu enters a power down state ? waiting for a external signal to clear does not (in my book) equate to shutting down.

I see no problem in doing what you want, the web page you pointed to seems to cover it well enough, I'd agree with Michael however that you need to examine how many writes to EEPROM you are going to make over the lifetime of the unit so that you do not exceed the lifespan of the EEPROM which is limited.

You could setup your code so that it checks how many writes have been made to a specified EEPROM block, and then switch to an alternative block and so on.