The feature is supposed to be supported, but even when following the official docs as closely as possible, I can't get passthrough working with UE on my Meta Quest 3S.

I enabled developer mode and tried both on UE5.6 and UE5.7:

1. Create a project from the VR template
2. Add this to DefaultEngine.ini and check in the console at runtime that these have been taken into account

   r.Mobile.PropagateAlpha=True
   r.PostProcess.PropagateAlpha=True
   xr.OpenXRInvertAlpha=True
   

3. Run the VR template map
4. Enable the passthrough at runtime on the left hand menu: nothing happens

I also dug into the what this button actually does and it does the following checks:

1. Calls IsCompositionLayerInvertedAlphaEnabled: should be true, returns false
2. SetEnvironmentBlendMode to Alpha Blend: does not throw, but when looking up GetEnvironmentBlendMode it returns Opaque instead of Alpha Blend
3. GetSupportedEnvironmentBlendModes: should return an array that contains Opaque and Alpha Blend, only contains Opaque