Found a really good article on how to prepare for emergency sanitation over at Self-Reliance Central. One of the suggestions is to have a bucket with a toilet seat, bags and kitty litter. And if you already have a cat, it's no biggie to buy an extra bag every time you pick up some up. Whoever uses the kitty litter, it is disposed of in the same fashion.
Of course, if you live out in the country, it might be prudent to locate a spot where you can dig a discreet outhouse disguised as a garden shed. There are so many fake outhouses out there, no one will give it a second thought as long as you are stealthy about digging the hole and scattering the dirt around the garden.
For that matter, it occurs to me that until you actually need to use the hole for matters sanitary, it could very well serve as storage. Just a nebulous thought. The details I leave to your imagination.