Only the person who authored FIN 2009 - G001
actually believes that a bank can verify
the percentage of revenues that a customer might generate from ineligible activities. In a prior century IRS Publication 1387 (I have a copy because rlcarey gave it to me
) contained a Model Customer Exemption statement. The one linked above is homemade, but better than the one originally published by the IRS.
If your reviewers will not accept a customer signed statement, you've given it your best shot. Just revoke the exemption, start filing the CTRs, and see how they like them apples.