I am not a materials expert at all, but maybe this will help:
On the Dash 8 I fly, we have heated elevator horns. The horns are a part of the leading edge of the elevator which protrudes into the airstream. Because of this, they are very susceptible to icing. You can see the horns clearly in the below photo on the outboard side of the horizontal stabilizer:
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Photo © Mark Kisiolek
The black part of the horn is not a boot, but an electrically heated segment. It is not metal, but fiberglass (I believe... could be wrong). I would imagine that those composite parts of modern jets that need to be protected with anti-icing systems are protected in the same manner, with electric heating elements.
Hope this helps.