We do not consider an authorized signer to be an account holder nor a customer for purposes of MI purchase. Our policy is that we do not sell to non-customers except in limited circumstances. We spell out the limited circumstances, one of them being that we will sell to a signer when the purchase is on behalf of the account owner (think signers on business accounts).
The remitter is always listed as the account holder in these instances. The MI log (when needed on cash purchases) is completed with all the required non-customer information from the actual purchaser (person at the window) with an extra line for "purchased on behalf of (insert customer name and account number)". Just came through a BSA exam, and had no problems with that method.
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I can't herd the cats anymore, so I just set up the electric fences and let them fry when they stray out of bounds.