Transporting a Carbon MTB
I don't know the official recommendation, but the clamp could be problematic with a carbon DT. 

Here's the thing, though: I don't think the clamp is strictly necessary. The purpose of the arm is to keep the bike from swaying laterally - you'll accomplish that as long as the arm is somehow attached to the frame. What I would do is put the DT in the clamp, but don't tighten the clamp: instead, use a rope or strap to tie the arm to the DT.

Another option: don't use the arm at all. Instead, use ratchet straps on either side to keep the bike upright. A lot of people do that when transporting motorbikes. This method seems more secure, but it might be difficult without a second set of hands.