Every time I have emailed for the contact deatils of a claimant I have received a response containing the registed email contact for the claimant. On one ocasion I had a number of claima and asked for contact details for all of them. For the disputes that had been rejected, Youtube gave me the contact detail. For the dispute awaiting response, I was informed that I would have to wait until the claimant repsonded before Youtube would release the contact details.
I only included the request for that specific information in my email to Youtube. Emails that do not conform to a DMCA notice or counternotice or a request for claimant details are rejected by the Youtube email filtering. I usually write "I am writing to request the contact details for all claimants on my video(s) <Video title> <Video ID>."