It has been reported that they can do that since the hold is not really a booking until payment is made. I do not have personal experience but some here have said in the past that they had been successful in doing so.
No, placeholders can only be applied to new bookings over the phone, a hold is considered an existing reservation, so by doing a hold, you are merely taking cabins out of the inventory at the fare at the time you make the hold, possibly eliminating availability in the cabin category and/or that lower fare.
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