When DJI did not decide to add INAV and Ardupilot to the list of the supported flight controller for the HD FPV system, some people were, at least, disappointed. Long story short: DJI OSD works only with Betaflight. Period.
Until today, DJI avoids the answer to the question if INAV and Ardupilot will be supported. I know there are some talks and there is a chance for native support for INAV and Ardupilot, but the timeline is not revealed.
In the meantime, there is a solution. It's based on, dadam, Arduino! Yeap, Arduino to the rescue! Thanks to Mavlink to MSP bridge DJI AirUnit thinks it's talking to the Betaflight flight controller, while indeed it is talking to any flight controller software that supports Mavlink! Since INAV supports Mavlink, you are golden! Enable Mavlink on one of the ports, connect DJI AirUnit via the Arduino and that's all!
It might not be the perfect solution to the problem, but it is a solution that seems to be working!