How to get translucent material to write alpha to frame buffer?

I’m using the Oculus Passthrough API, which looks at the alpha channel of the frame buffer to determine how much to blend in the headset cameras. The translucent/additive materials I’m using have an alpha value of zero in renderdoc, and therefore are not showing at all in game.

How can I get translucent/additive materials to write their alpha to the frame buffer?

Platform is mobile vulkan (quest 2). Thanks!