It's a bit of a grey area I agree, but I think 'passenger-honesty' is probs the key here.
As far as I'm concerned.The guitar is paid for is now used and is no different to me picking up at an uncles and taking it back to the UK.
Will defo be pushing for getting it on the plane though.Its nice to hear that airline staff have stored it in the wardrobes for people in the past.