A work-around: I bought one of those external battery-powered intercoms
some time ago, for using in 4-seat rentals with 2-place intercoms.

It has a line-in and line-out jack.

Now I plug it in to the back seat headset jacks (and can plug a headset
(or 4, for a total of 7), and record audio though the line-out jack,
with NO performance degradation of the rest of the system.

It's expensive, but I already spent the money, and it works.