You'd have to check with the airline about this one. I would guess that they will say no for safety reasons. Definately for take off and landing, baby will have a seatbelt extension from mum and will have to sit on her lap. Same goes for turbulance (quite a pain when they are happily snoozing). Is there no way you could spring for another seat? I think it would be well worth it. We are going in a few weeks and have booked a seat for DS11mths. Virgin actually provide a carseat type seat for him and he can be independant from us. Might be worth checking if Travel City have a similar thing. Also, it would just give you room to manouvere when baby falls asleep etc.
Sorry I can't help more but anything i can do, just ask,
Claire