However, nothing I've found from IRS sources confirms this, and in fact what I've found indicates the opposite.
This is from Rev. Proc. 2011-47:
This revenue procedure provides rules for using a per diem rate to substantiate the amount of an employee’s expenses for lodging, meal, and incidental expenses, or for meal and incidental expenses only, that a payor (an employer, its agent, or a third party) reimburses. Employees and self-employed individuals that deduct unreimbursed expenses for travel away from home may use a per diem rate for meals and incidental expenses, or incidental expenses only, under this revenue procedure.
The part about reimbursements indicates it only applies to employees. Can a partner of a partnership be counted as an employee for this purpose? Probably not.
The part about deductions indicates it applies to "individuals that deduct unreimbursed expenses". A partner that submits a reimbursement request doesn't fall into this category because the expense is not "unreimbursed". Likewise, a partnership with expenses to deduct doesn't fall into the category because it isn't an "individual".
This is from Publication 463:
As a self-employed person, you adequately account by reporting your actual expenses.
Although the quote is from a section about independent contractors, the statement itself is broad and would seem to apply in any case where the expenses are incurred by someone other than an employee...such as a partner in a partnership.
Of course, none of this would stop a partner from taking the SMR, per diem, or simplified home office deductions as unreimbursed partnership expenses.
And that leads me to my final thought...can a partnership agreement provide for guaranteed payments in respect of these UPE's? Section 707(c) only affords guaranteed payment treatment "for services or the use of capital". It doesn't seem like paying expenses that aren't partnership expenses would fall into either of those categories, but I don't know. If guaranteed payments can be computed this way, that would produce the same result as if the SMR, etc. had been reimbursed by the partnership. It could also be used to allow some partners to circumvent the § 280A(c)(5) limit on the home office deduction.