Actually, I was able to get it working. 
You can set up your Blueprint like so, or use the equivalent C++ functions:
This Blueprint adds the CesiumPolygonRasterOverlay component to the desired tileset. Then, it sets the CesiumCartographicPolygon array that the raster overlay should use. Finally, it refreshes the overlay so that the new polygons take effect.
Let me know if you have any questions!