We used to purchase an extra seat and strap the baby carrier to the seat using the seat belt. The only stipulation was that the carrier had to be attached to the window seat so it wasn't in the way in case of evacuation. The one issue we had was that we knew more about what was allowed than many of the cabin crew, usually the 1st crew member we met said it wasn't ok, only after speaking to a more senior member did they come back and admit it was ok.