Driving around Ireland the other day, heard the RTE1 radio station call in with a guy who has been making these jackets for sale.
Then the Ryanair director of communications (or sumsuch title) phoned in, was very positive about the jacket and said the airline had no problem at all with it, as long as it fits under the seat in front of the pax.
But if it has to go into the overhead with WITH the pax's carry-on, using up more space, then that's not okay. Then it becomes a second carry-on.
But otherwise he was pretty funny about it and said he thought the jacket was a great idea!
It caused a bit of head scratching among the radio presenters, but it made sense. The Ryanair employee said it's the overhead bin space they are focused on more, so as long as the jacket doesn't use more of that, then they don't care if people have them.
What the other airlines think, who knows....