I know that it will depend on the language in the power of attorney document itself, but are there any prohibitions against having the POA (who is a borrower) sign regulatory disclosures on behalf of her husband (who is the other borrower) on a loan refi (or any loan for that matter)?

Like you said, depends on the type and wording of the POA, and state law. (Some states, the POA must be a registered document for transfer of real estate.)

That being said, that is precisely the reason for the POA.