BA will definitely do it for cash!
I missed my JFK-LHR flight a few months ago due to really bad traffic (and my stupid decision to use a car service instead of the train, but I digress). I didn't actually miss the flight - I had 15 mins, but I think the gate had already given away my seat.
Just as I was about to buy a one-way ticket...
A BA angel decided to accept my endorsed ticket, simply because someone from UA had done something nice for one of his pax the other day. The UA folks printed it out, and I took it over to BA. They issued a BP. Thereafter, I asked if I could buy up - they let me buy WT+ ($250), but not C because there was no FFCC avail for that flight. C was 90% empty.
So, BA is perfectly willing to accept the buy-up revenues. Not sure if they would do it for miles or miles & cash, but all cash is definitely cool.
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