I arrived back in the UK last week on BA and the flight was so full they gave the jump seat to a pax. Gutted that it wasn't me who was the chosen one I was still very surprised that this was allowed. I was one of the last to get to the gate a overheard that they would have to use it so don't think it was pre arranged by a relative or friend of staff.
Might as well ask every time you fly mate - if you don't play you can't win.