Planting on a Steep Slope in Zone 5b

I live in Kingston Ontario, Canada and have a fairly steep slope in my shady back yard. The yard is facing North West. Shade is provided by deciduous trees (light to dappled in spring and fall; dappled moderate in summer). Soil is clay-based, which we have spent 6 years amending with leaf mulch annually. We back onto conservation land and have an existing garden nearby that is focussed on native species and some exotics geared towards attracting bees, butterflies and hummingbirds (honeysuckle on the wire fence, bee balm, catmint, Joe Pye weed, etc.) Grass grows on this slope but it is dangerous to cut! We would like to replace the grass with plantings that are dense, low, and wild-life and environmentally sensitive as we are aware of the proximity and needs of the Conservation Land onto which our property borders. Suggestions?

Since grass does grow on your slope, it cannot be too shady and does leave you some choices. The best choice would probably be crown vetch, but it is not native and does seed around a little bit. Adjacent to a conservation area, you might want to give it a pass. How about the Heucheras? There are many great colour forms and the parental species are native. Some of the native clump forming Aster species might also work, but they are not low growing. They might be an idea for areas where you have trouble keeping the soil in place. And the odd high plant adds variety.
Have you considered making the area a shady rock garden with lots of mulch (bark is better than leaves, since it does not blow away). Hepaticas (P3736) would be great, since they keep their leaves all year. Even Semperviviums (Hens-and-Chickens) would succeed in the brighter areas, if they have no competition. And they do not seed around. Some thymes (lots of choice from Richters) would be lovely in brighter spots and they do not seed around. Again you would have to eliminate competition for them. Another great native is Anemone canadensis with its mat forming habit and 2cm white flowers in early summer. A higher plant is yellow bedstraw (P1335). I have found that it does not seed readily but forms mats with great gusto. In shade it will probably try to creep into the sun.
If the soil is moist enough, a few Hellebores would add a great foliage contrast and early long-lasting bloom. They are not native but rarely seed at all. You already have Joe-pye weed which likes moist soil, so there is hope for Hellebores. Have you considered some taller accent plants of the brown-leaved Eupatoriums? Richters offers a nice one, chocolate boneset, catalogue (P1467-500). It too does not like competition. A mildly aggressive native groundcover would be mayapple, (P3960) in Richters catalogue.