as you know you can use the displacement modifier and apply it only to a vertex selection. This way the displacement effect blends from displaced into not displaced nice and smooth.
Now is there maybe a way to do this directly inside sculpt mode like using an alpha like map to restrain where you can only sculpt and the map describes the blending between where you can and cannot sculpt?
Here you see an image of a mesh. Imagine the lower left edge should be the original mesh contour blending into the sculpted texture.