Is there anywhere I can find examples of letters seeking interpretation of FARs, sent to the FAA Office of Chief Counsel?
The FAA posts the original submission along with the interpretations, at least for recent interpretations. There's a lot of variation in who they're addressed to; many of them are follow-ups to previous contact with the FAA, presumably with the local FSDO. 

This one (Standel-West Coast Aviation Services, 2019) might be useful as a starting point, it simply uses the salutation "To Whom It May Concern" and it has the full mailing address in it. It also uses very specific wording about the scope of the request:


  This letter is a request for interpretation and clarification of 14
  C.F.R. § 135.267(b).