Low voltage disconnect with no quiescent current
You could use a micro power comparator like the LTC1998 to control the EN pin. That particular IC would draw 2.5 uA which is of course more that the standby current of your regulator. But If you do the figures for the battery life it will last a pretty long time.

There are other comparators with even lower current draw, but the packages might not suit you if you're a hobbyist. 

LTC1998