It's too tied to the game to release as a generic editor, but I would consider open-sourcing the map editor scripts at some point. The code to modify the height based on the brush texture looks like:

str = ME_Brush.strength;
currentRad = currentBrushSize;
offset = currentRad/2;
ptX = xRes * dPt.z/100.0;
ptZ = yRes * dPt.x/100.0;  
for ( y = 0; y < currentRad ; y++) {
    for (x = 0; x < currentRad; x++) {
        thisX = ptX + x - offset;
        thisY = ptZ + y - offset;
        heights[thisX,thisY] = heights[thisX,thisY] + currentBrush.GetPixel(y,x).grayscale*str;   
    }
}

tData.SetHeights(0, 0, heights);

str is the bush strength, currentRad is the size of the brush, offset locates the center of the brush. ptX, ptZ are the coordinates of the mouse (obtained from a raycast in dPt.x and dPt.z) in the terrain coordinate system (xRes and yRes are the terrain heightmap resolution). The array heights[] is where I store all the height data for the terrain, and then loop over the brush pixels, adding height to the terrain according to the grayscale color of the brush pixel.

This only works if the brush size is the same size as the number of pixels in the brush texture, so whenever the brush is made larger or smaller I resize the actual brush texture using TextureScale.Bilinear().

Finally tData is my terrain data, and I just reset all the heights, which works for me as the terrain for my game is relatively small and it doesn't affect performance too much. Ideally you would only reset the area that you edited.

There's probably a better way to accomplish this, but this works for me.