[Gear VR] cubemap Stereo Layer with alpha - possible ?

I was brainstorming a cockpit game for Gear VR (racing, flying, etc.)  and I think that one of the ways to have highly detailed cockpit without sacrificing polycount budget for the scene is by rendering stereo cubemap of the cockpit with alpha channel for windows in a 3D app and having it rendered in the cubemap stereo layer in the game. 

Not really sure how to set it up since stereo layer needs a texture, not a material and I don't recall seeing cubemaps with alpha.

Is it achievable ? How would I set it up? 

Thanks