How about a cat flap with a wire mesh cage/tunnel so they can go out, but be safe?

If I remember your backyard, it's quite big, so you can create any size shape you want either using chicken wire type fencing or something stronger, like this:

PurrfectFence

TheCatsDen

I wouldn't trust my two outdoors at all. Even with Nakita, I always went out with a leash and harness since there are too many distractions for a cat.

Hope Zazou comes back soon!