Other Has anyone tried to roll custom solution instead of Cardboard?

I've run into a couple of limitations of current opensource cardboard. Namely I can't add ui elements to the 2d overlay, can't switch between flat and vr mode normally.

That got me thinking.

Is there a good reason not to replace cardboard's behavior with custom framework? I mean, I can render the the scene twice for each eye and apply lens distortion.

I'm mostly contemplating limited use cases where application is tailored for a single device or a small number of them.