With the current WebVR API, I can get headset position & orientation data relative to the zeroState.
But how can I determine where the headset is relative to the positional tracker camera? I'd like to draw a 3D model of the tracker along with FOV constraints in the virtual world, much like the Oculus Config demo scene.
WebVR does not expose the positional tracker camera via API, and they do not have any plans to expose it as other headsets might not have external cameras. Therefore, it's impossible to determine headset position/orientation relative to the tracker, and there is no frustum data.
"At this point no, because it's not obvious if that will be a common element of many HMDs."