Reg DD refers not to the last 7 days interest the bank paid, but to a quantity/amount. From the definitions in Reg DD:
"means an account with a maturity of at least seven days in which the consumer generally does not have a right to make withdrawals for six days after the account is opened, unless the deposit is subject to an early withdrawal penalty of at least seven days' interest on amounts withdrawn."
So if the consumer wanted to withdraw the funds at day 5, the bank would take more interest than accrued and that would be principal. So this is already done.
If your product pays the interest immediately, you would calculate the daily interest and deduct the disclosed amount (based on the days) which may be greater than 7 days worth. Interest paid is irrelevant, the accrual based on the principal and rate are what the formula is based on. There is no cap and a bank may be wise to consider a stringent penalty to dissuade those thinking they could make a quick buck leaving funds with you for only a week.