6-axis quat fusion with raw magnetometer data (no MPL)

Does anyone have an effective method for fusing the magnetometer data with the 6-axis quaternion that comes from the DMP without using the MPL?

I have code that provides me with the 6-axis quaternion (from DMP) and raw magnetometer data. I know how to calibrate the magnetometer myself, but haven't found a good way to adjust the "6-axis quaternion" based on the magnetometer data to get a "9-axis quaternion" with improved heading stability.
SDK 5.1 does this, though not perfectly, but the source code is not available.

I am hoping the platform independent release will contain this information, but I can't wait forever for it.