Hey! You can easily access the layer in a custom post-processing task. The "Run" method receives a "GeneratedLevel" object and on that, you can call the "GetSharedTilemaps()" method and it gives you only the shared tilemaps for the whole level. In this list of tilemaps, there's only a single one with the name "Walls" - the one that you're looking for.
Let me know if that helps.