Leaves can work fine, if they're large and green and can be folded/bunched without crumbling.
When I dip off the road into the underbrush here in north Mississippi, the first thing I look for is two or three sticks of 1/2'' to 1" in diameter. Sometimes I'll break a long one across my knee to make two. You wipe (and twist) with one end, then drum-majorette the thing in your hand and do the same thing with the other end. Two sticks, four wipes and you're mostly there.
I've never used a rock, but I'll keep that in mind. In fact, I'll never hear the phrase "Let's rock!" the same way again.
As for moss: WFT?!?! Almost guaranteed to leave your bum covered with prickly little bits. Never would I do that.
Those who claim they've never had to pit-stop to take a s--t during a run probably aren't eating enough roughage.