An agent acting under a power of attorney for an individual who has rights to own, possess, or use NFA firearms as an individual or in a NFA Firearms Trust, may not take possession, hold, use, or have access to the NFA firearms.
There is only one situation when an agent may do so. After the death of an individual who has rights to or owns NFA Firearms individually or who is current beneficiary of the NFA firearms, the court will appoint a Personal Representative. The court appointed Personal Representative only and not one who is only named in a will, may have access, and transfer the NFA firearms within a reasonable period of time. There is no guidance on what the ATF considers reasonable, but I would suggest starring the process as soon as possible so that the definition of reasonableness is not determined in a case involving you.
The easy way to deal with this for those of you who own Firearms in a NFA trust is to modify to the trust to allow the person who is the agent to be a trustee in the trust. Remember you can add or remove people from a NFA Trust at any time with a simple amendment.