You would have to contact IB directly. I've never had luck doing so--they just don't "stick" in the system. When I have tried, the seat assignments usually revert to "unassigned" within a few days.

The business of pre-reserving seats is not very common in Europe. Most passengers have their seats assigned upon arrival at the airport on day of departure--it's the culture there. Also, it allows the airlines flexibility to do equipment swaps, and trust me, they do so a LOT. Whenever you are prevented from checking in (either online or in person), and you inquire, chances are the answer will be "oh, our back-room staff is 'working on' the flight, so you can't check in now," meaning that an equipment swap is in progress.



On the other hand, I like to pre-reserve my seat when I book the ticket nine months in advance. That way, I will be assured of getting my favorite seat and being at the correct end of FEBO (if applicable). So, I try to fly AA metal so I can pre-reserve.