Yeah, OVRManager.monoscopic is a global setting, I wouldn't recommend using this.
To render a camera in mono, parent it under the center eye anchor, set the "target eye" to "none" and set a target texture.
We actually have a demo of doing hybrid mono/stereo rendering in Unity. If you set the disparity of the mono images correctly, the transition from mono to stereo is seamless. Hopefully we'll release this soon.