Could a bank use the "New Account" hold reason if the only other account the customer has with the bank is a savings account? Reg CC's definition of an account does not include a savings account, and is clearly exempted in the definition. Why would the definition of an account be different for purposes of the hold reason?

Yes, you have a new account if the prior relationship did not involve a transaction account.

From the commentary:
"iii. If a customer has a savings deposit or other deposit that is not an account (as that term is defined in §229.2(a)) at the bank, and opens an account, the account is subject to the new account exception."