For putting on a clone of the head bone and using it as an IK target in VRIK.

Unity/OpenSeeIKTarget.cs

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+using UnityEngine;
+
+public class OpenSeeIKTarget : MonoBehaviour
+{
+    public OpenSee.OpenSee openSee;
+    public bool calibrate = true;
+    public int faceId = 0;
+    public float translationScale = 0.3f;
+    public float smooth = 0.2f;
+    double updated = 0.0f;
+    Quaternion dR;
+    Vector3 dT;
+
+    void Update()
+    {
+        var openSeeData = openSee.trackingData;
+        if (openSeeData == null || openSeeData.Length < 1)
+            return;
+        int idx = -1;
+        int i = 0;
+        foreach (var item in openSeeData) {
+            if (item.id == faceId)
+                idx = i;
+            i++;
+        }
+        if (idx < 0)
+            return;
+        if (openSeeData[idx].time > updated) {
+            updated = openSeeData[idx].time;
+        } else {
+            return;
+        }
+        
+        if (calibrate) {
+            calibrate = false;
+            dR = Quaternion.Euler(-openSeeData[idx].rotation.x, openSeeData[idx].rotation.y, -openSeeData[0].rotation.z);
+            dR = Quaternion.Inverse(dR);
+            dT = openSeeData[idx].translation;
+        }
+        
+        Vector3 v = openSeeData[idx].translation - dT;
+        v.z = -v.z;
+        transform.localPosition = Vector3.Lerp(transform.localPosition, v * translationScale, 1f - smooth);
+        transform.localRotation = Quaternion.Lerp(transform.localRotation, Quaternion.Euler(-openSeeData[idx].rotation.x, openSeeData[idx].rotation.y, -openSeeData[idx].rotation.z) * dR, 1f - smooth);
+    }
+}

Unity/OpenSeeIKTarget.cs.meta

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