Is there support for an external compass (mag with imu) instead of just an external mag? Seems like a way to get a cleaner heading solution.
Why could not you use onboard imu?
We currently are but we've noticed that if the external mag is offset slightly in z-axis angle we start to get errors. We're trying to get the most accurate heading/attitude data b/c we have some sensors we're trying to geolocate to the ground.
Perhaps then you can start with frame transforming? transform onboard IMU data to where your external mag located and fuse them in advance.
An external IMU can work, but there's nothing in place to treat that data differently than the internal sensors. External mag is always preferred though.