Aside from specific requirements to disclose it after the fact I'd like to hear what anyone thinks about the UDAAP risk. Adding the tax payment to principal means it accrues interest and becomes secured by the borrower's home. This could cause financial harm, so it seems like it could be unfair under UDAAP if you don't inform the borrower of the impact in advance and give them a way to avoid it.
Unpaid real estate taxes are also a lien on the real estate, and it's a lien senior to your loan in most states. And at least where I live, unpaid tax bills accrue interest -- sometimes at rates great than the mortgage loan. So I don't see a financial argument here. I'm only concerned with whether the state law will allow it, and the loan or mortgage documents provide for it.