I received an 1099-R from a retirement plan in the Estate's EIN. Is there a way to get the withholding to flow to the K-1 beneficiaries so that they get credit ?

The treatment is codified in §643.  Allocation of withholding to beneficiaries is limited in its application to subsection (d) for backup withholding governed by §3406 and, by election, under subsection (g) for estimated tax payments made by the trust.

You are not allowed to allocate regular withholding to beneficiaries.  The only exceptions would be (1) where it is a grantor trust and (2) where a person other than the estate has the right to receive amounts of IRD.

§31(a) stipulates that amounts withheld as tax under chapter 24 of the Code "shall be allowed to the recipient of the income as a credit against the tax imposed by" subtitle A (income tax).  Since the trust is subject to income tax under §1(e) with deduction for DNI, any such credit is allowable only to the trust as §31 and its regulations do not provide for any mechanism for allocation to beneficiaries.  This position is clearly stated in CCA 200644018 and in the filing instructions for F.1041.