On a Representative Payee Account for Social Security payments who would be the customer for CIP purposes, the owner of the funds, the representative payee or both?

The representative payee. See the CIP FAQ HERE.

Look for the first question in the section on the definition of "customer" (near the beginning of the document). In the case of a Representative Payee account, the beneficiary of the Social Security payment does not have the legal capacity to open the account (one cannot open a Representative Payee account for oneself).

The guide at the url below is also useful, particulary pages 5 and 13 dealing with POAs and account titling.

http://www.ssa.gov/pubs/10076.pdf