As @Karen_pdx states, "My understanding has always been that a trust document's provisions can't override the IRC."

There are two kinds of allocations.  One allocation is between trust and beneficiary, and usually involves capital gains.  Trustee can allocate them to the beneficiary, which often saves tax, or to the trust, which is usually the default.  IRS doesn't care who reports the income, as long as it gets reported.

And then there is allocation of deductions between taxable income and tax-exempt income.  That's required by the law of the land, not by the "law" of the trust document. 

Maybe you could allocate all of the taxable income to the beneficiary, and therefore all of the deductions.  But you can't allocate some of the tax-exempt income, and none of the deductions related to it, to the beneficiary.