Originally Posted by dannyr
Not having tried myself, but I'd suggest the best way to pull this off would be to offer the cash to someone who appears as a business traveller (eg they didn't pay for the ticket themselves) and simply swap boarding passes with them.
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This reminds me of a friend who used to fly in Biz from the US to Brazil quite often. He would prowl the back of coach for anyone lounging across the 4-seat middle section (on a less than packed flight), and he'd offer someone a swap - his biz seat for theirs. He just wanted the flat row of seats to sleep, before lie-flat seats started becoming more common. He never failed to find a taker.