No fix for it since Oculus team replied to my email that this feature doesn't exist, a big shame to hear that,
auzden shared a workaround to add this feature but it will require source engine modification and only works on Vulkan. not sure about the performance impact or any other side effects. so do it on your own risk.
I ran into this problem too. While playing around with things, I edited my Widget BP's design to have a transparent background, instead of trying to have my material control the opacity, and the transparency for the Widget's RenderTarget correctly shows up on as transparent.
I hope that helps someone!
I'm still fighting two losing battles. I want my stereo layer to still do depth testing, and that isn't working for me, as it always renders on top of everything. I'm not sure if that is by design, current limitation, or I am just missing something. While trying to get it to work, I noticed that I can't get the material used by the Widget to ever affect the Stereo Layer.