Preserve Normals with Mask Modifier

I am working on some game assets, basically a set of cliff pieces that snap together when moved on grid alignment. In order to get the geometry to line up, I created some extra geometry past the edges, which gets removed by the mask modifier. However, while the geometry lines up, the surface normals do not. In order to get them to line up, I must make a copy of each piece, which lacks the mask modifier.

If the Mask modifier were extended with an option to preserve the normals along the new edges, it would make this work a bit less tedious.