navsat_transform_node without IMU

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I'm trying to fuse GPS data and IMU data using robot_localization. I have the IMU data going into the filter nicely, now I'm trying to configure GPS. The problem is that my robot has a large inverter relatively close to where the IMU is mounted which is causing too much magnetic noise, so the magnetometer on the IMU is disabled. I understand that the navsat_transform_node requires an absolute starting heading (usually from the IMU) to initialize properly. What are some ways I can get around this?

I thought about enabling the "wait_for_datum" parameter and then using the GPS to manually using the set_datum service. Has anyone done this in the past?