Sedums! There are dozens of varieties that would even thrive in the conditions you described. I would look for s. reflexum, s. oreganum, or similar low-growing varieties that spread by runners. Over a couple of years, these will form a nice dense mat that will hold itself in place.

Many sedums grow happily on sheer rock cliffs where you'd think nothing could hold on... but they do.

Google around for "sedum" and find a few varieties that tickle your fancy. If you are the frugal sort, don't get a lot of plants... you can divide even a 4" pot into several plants and they will just take a little longer to fill in. I prefer to spend my money on variety rather than volume, so I get several different kinds, but only one pot of each.

Do double-check the hardiness, however anything you buy from a reputable nursery in your area should grow in your area.

Here is a link that might be useful: Sedum photos

The wine-leaf cinquefoil grows naturally in such difficult spots. I know it is a Potentilla, but the exact species eludes me, at the moment. P. tridentata? What about Boston Ivy. Boring in Winter, perhaps too vigorous, nice in Summer and unbelievable in Fall. It could be rooted at the bottom and grow upwards. Lowbush blueberry, V. angustifolia, grows in crevices in Maine.