My front walkway is of the dry stack variety -- there's no mortar between any of the stones. Stone dust was tamped in during construction, but with each heavy rain, a bit more of it washes down the sloping yard.
I got the idea to plant some hens and chicks in the spaces that have lost the most rock dust when I saw them in another stone walkway. There was already some decaying organic matter in the cracks: broken down sticks, leaves, nuts. I just broke a piece off the hen/chick pile and crammed it into a few spots.
Plus they're cute, and they do seem to be helping to create some hold in the risky patches of the walkway.