Anyone had a customer request a notation printed on their personal checks that would prohibit electronic processing? Was the request honored?

That would be an exercise in extreme futility. Suppose that you permit the customer to put that legend on checks. Then suppose that your customer issues a check to XYZ Cable Company to pay a monthly bill, and that XYZ Cable overlooks the check legend and converts the check to an ARC entry. Will your bank have the ability to flag at the account level ARC, POP, BOC and RCK entries? When you flag them, will you send them back on Settlement Date +1? There is not an appropriate Return Reason code (as far as I can see) for "not authorized" except R10, which requires a WSUPP. The R39 (Improper Source Document) code doesn't cover "not authorized" situations.

Of course the main problem is that your customer will blame YOU, not the check payee, for allowing the conversion.