I hate to keep quoting the ERISA Outline book (actually I don't), but I found this in Chapter 6: Plan Distributions - Section V (Death benefits)
1.c. Power of attorney. If a person holds a power of attorney (POA) with respect to a plan participant, can the POA change the participant's beneficiary? This was the issue in Clouse v.Philadelphia, Bethlehem & New England Railroad Co., 787 F.Supp. 93 (E.D.Pa. 1992). The case involved a general power of attorney. The court concluded, in reliance of Section 37 of the Restatement (Second), Agency, that a general power of attorney did not authorize the POA to change the beneficiary designation. A more specifically-drafted POA was needed. Also note that the terms of the plan document need to be consulted to ensure that the plan's procedures for changing beneficiaries are followed.
You may have already seen this, but I thought I would throw it out there anyway.