If your island is made of metaballs, and you want to use the terrain-at-once technique, simply switch to the isometric overhead view and zoome out until your island fits the screen, adjust the render size to 1024x1024 or 2048x2048, and render as an Object Mask. Import the resulting image back into Bryce as a terrain map, and boom, instant matching edges... Also, you can of course use this technique to make rolling waves all day in Photoshop. Then re-import the image into the Terrain Editor and boom, you're the man.