Well, the hidden feature seemed pretty handy to me anyway.  I've not seen it mentioned anywhere, not even by DJI.  All is revealed in this mercifully short video.... together with a success in connecting a lav mic to the camera with a readily available USB interface.

Hello there Peter. Good day and thank you for posting another informative video you have filmed and created. Nicely done and thank you for your support.

Thanks for sharing.

Thanks for sharing

Nicely done.  Thanks for sharing.  

Very nice and definitely a cheaper alternative to the wireless microphone option.

Nice but what if you want to record for a really long time with a external mic?   
There are many use cases, the biggest being fishing including Kayak Fishing, it's a huge market not to mention regular boats.
We really need a USB adapter that allows a TRS 3.5mm mic and pass thru power.