Is there an exemption in CRA, similar to HMDA, where the revenue of a business owned by a bank director or employee would not need to be reported, i.e., use code 3 for "unknown" rather than the code for the actual revenue used to make the decision?

No there is no exemption. Revenues have nothing to do with income, so no privacy issues will be of concern here.

Use a 3 when you have used revenues on a loan to a director and you're going to A, be in a data integrity bind, and B, indicate to your examiners that you offer loans, not based on any financial information, to your reg 0 folks. This is a red flag for reg 0.

Use the correct code.