Are contractors required to include a cover letter with their CDRL data deliverables? Does the requirement for a cover letter need to be spelled out on the CDRL, contract, or statement or work? Is there a FAR or DFAR clause regarding cover letters?
Neither the instructions for DD Form 1423, Contract Data Requirements List, or the DFARS PGI stipulate a cover letter. DoD 5010.12-M addresses technical data requirements and does speak to some best practices employed by organizations working with data packages. Although cover letters weren't specifically mentioned in DoD 5010.12-M, it is similar in nature to several of the best practices that were listed.
Regardless, the requirement for a cover letter needs to be an explicit term and condition of the contract. Although you've already checked; the contract, the SOW, and block 16 (remarks field) of the DD 1423 are all places to look for those types of instructions. I don't have access to the ASSIST database that maintains Data Item Descriptions (DID's), but that would be another place to check for formatting requirements on specific data deliverables. The only other consideration would be if the medium for delivery, say a system like WAWF/iRAPT, has specific requirements for attachments.