Does the custodian have contractual authority to file the 1099R? If it files the 1099R's per direction from someone else per its service contract, it can notify that person and await instruction.
Generally, a custodian is at least a full step down from a directed trustee. Per its service contract, there should be no authority or ability to act in the absence of direction from the trustee or plan administrator; therefore, there should be no legal liability to act. In theory, it lacks sufficient information to act, the trustee could be depositing loan repayments in another account held by the trust.