Take a look at 537.2503 for information on deferral fees.
We offer a similar program, similar time of year, with great success in generating fee income. This has been reviewed by examiners more than once, as well as external legal counsel. Don't add the fee to the loan, don't charge more than you're allowed to, get signatures with verbiage acknowledging that you're essentially modifying the loan, and be careful if you also allow extensions on your closed-end loans as too many skips could be considered "hiding delinquency", or that was the vague guidance we were given by our State exam team.
Someone's about to get horned!