Nothing in ERISA’s title I requires a plan to include a provision for recognizing a disclaimer.
In my experience, the IRS’s tax-qualification reviewers express no objection to a document’s detailed provisions for recognizing a beneficiary’s disclaimer and setting conditions on a disclaimer the plan’s administrator will follow.
An IRS-preapproved document might lack those provisions.
It is unclear whether one could add those provisions without defeating a user’s anticipated reliance on the Internal Revenue Service opinion letter on the preapproved document.
A plan’s administrator must obey the plan’s governing document. ERISA § 404(a)(1)(D).
Although a document might grant the administrator some power to interpret the document, it is a power to interpret ambiguous provisions, not to rewrite the document.
An administrator might disobey the plan’s governing document, perhaps considering that the disclaimant is unlikely to sue on the fiduciary’s breach.
If an administrator allows a disclaimer, the administrator might recognize only a document that meets conditions under Internal Revenue Code § 2518. Although that section is in an Internal Revenue Code chapter about gift tax, the Treasury department treats a disclaimer that meets the § 2518 conditions as also effective to remove the refused property from the disclaimant’s income for Federal income tax purposes. Without that, a payer might face difficult questions about whether to tax-report a distribution paid to someone else as a distribution to the disclaimant.
The logic path above assumes neither A nor B is (or is deemed) a surviving spouse.