Something I did when trying to figure out a way to create the illusion that road intersections were connected to the driveway in my apartment complex was make a duplicate of the Dirt_Sidewalk_Opacity.DDS file and just made it completely black which had the effect of making any custom sidewalk that used that file completely invisible.
Then when I made my custom road I used that Completely black DDS file for the intersection sidewalk opacity.
You can see the effect that had in this screenshot...
I eventually figured out a better trick that didn't need the custom road intersection at all.
But the invisible sidewalk trick looks like it would work well for this problem too and those .DDS files are tiny in size. just a few k so it won't add any significant size to your world.