I've used the mike-in-earcup method successfully. One needs a mike small enough to fit in there entirely, so the acoustic seal is retained. It's hard to find a camcorder compatible mike that's small enough.
The mike will pick up everything the person can hear, including any aircraft noise that gets in directly, and gets in via the noise-cancelling mike not being 100% cancelling. An ANR headset is highly desirable.
A direct intercom connection would avoid the need to insert the mike in the headset but the performance won't be much better.
A completely separate system would still be only as good as the noise cancellation of the mike being used and that is the area where the pros spend the money.