How can I tweak Unity's implementation of VR?

I hear that since Unity 5.4, VR is handled natively in Unity rather than through plugins like the SteamVR Plugin. Where can I tweak this implementation? For research purposes, I want to temporarily lock HMD yaw when the player presses a button.

It’s not a great solution but you could counteract the yaw in the camera parent by getting the tracking data and inverting it.