haven't read the other replies yet so maybe this is mentioned...
If I really had to go coach, for a 5 day trip, anywhere in the world, and I'm being taxed on an inflated $20K worth of value, I think I would decline to accept the trip for tax reasons.
How can you spend $20K in genuine costs on a five day trip if they don't fly you out in business???
I guess instead of asking flyertalk, I would be asking my accountant how much this prize might cost me. Sorry to be a Debbie Downer, but there is a big difference between "having $20K to spend to go anywhere in the world" and, oops, it must be approved, it must be in coach, it must be 5 days! That makes it seem to me that they're not seriously intending to give you anywhere near $20K worth of value but you get taxed on what they put on that 1099. I would be very cautious before accepting this prize...