Hi there, Wave SDK engineer here.
As you mentioned, the API current provided by the SDK can only be used to adjust the opacity of passthrough overlays but not underlays, so allow me present you an alternative solution.
Provided that you have already succeeded in showing passthrough underlay in your scene in Unity, you should have either a skybox material or the main camera background color in the scene configured in a way, such that it clears the background of the scene to transparent black (i.e. RGBA = 0,0,0,0).
You should be able to achieve the same effect by adjusting the alpha value of the material or the main camera background color.
Hope this helps 🙂