Querying HMD Pose from Device Service

[Misomosi]

I am developing a Device Service for our hand tracking hardware for 6dof tracking. In order to get the fully-expressed pose for each tracker, however, it is necessary to query the HMD's pose as the sensor is mounted on the HMD itself. It certainly seems as if the Finch controller service does this, but there is nothing in the documentation of how a device service might get this information. The only context that is documented is the VR client app, but that's not what we're trying to make over here...

Any help would be appreciated.