There is nothing in the BSA regulations to prevent you from telling the customer why you are asking for the information.
As a practical matter, I cannot imagine how you'd get the information from most customers without explaining your interest, unless the customer happened to borrow from your bank and provided such detail (again, really doubtful) in financial statements supporting the loan.
John S. Burnett
Fighting for Compliance since 1976
Bankers' Threads User #8