Luke, if you perform a diligent search and are unable to determine the beneficiary of a decedent by the time you file the Form MP you would list the benefit under the deceased participant's information. (For example, let's say you find out the participant died years ago when processing the plan termination, you have no beneficiary designation form on file, and you either can't determine who the participant's survivors were based on the default beneficiary hierarchy in the plan document, or you can't find an individual administering his estate if the benefit is payable to the participant's estate),
The preamble to the 2017 PBGC missing participant regulations under section 4050 stated the following:
"PBGC expects that there will be instances where a DC plan knows a participant is deceased but has little or no information about a beneficiary. Where an unknown beneficiary of a deceased participant is missing, as defined in the final regulation, the account balance of the deceased participant may be transferred into the missing participants program. PBGC will take into account the fact that there is no known person to search for in evaluating the plan’s fulfillment of the diligent search requirement for any such distributee. Plan fiduciaries and QTAs would file in accordance with the forms and instructions for DC plans what information they have about the participant and beneficiary."