I would scarify the soil with a rake or similar tool. Make furrows or grooves in the soil perpendicular to the slope to slow water flow down the slope and to hold seeds. Scratches at least a half inch deep should be enough. You might want to moisten the soil a day before you try to make the scratches. Unless you add a lot (6 inches or more) adding topsoil probably isn't going to help much. Plants won't do well in a thin layer of soil over a much denser clay. I think if you scarify most of the soil surface, plant seed very thickly, and give it water every couple of days for the first several weeks to get the seedlings off to a good start, you should avoid major erosion problems. There will probably be some local spots where there is erosion, but most of the slope should be OK.