Try a 40db pad from a company called Canford Audio based in Washington Tyne & Wear Or you may get one from your local maplin.
If you tell them what you are trying to do they will have the right answer.
Most video cameras have an automatic level control built in but as mentioned before they can only cope with so much and full line level into the camera socket from the intercom would FUBAR the camera.
I have used the 40db pad lots and it gives great results.... I've also used the clip mic in the headset and that works pretty well too.
Let us know how you get on.
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