Just a further data point to add to this thread. I have an upcoming Avios booking on IB that was booked on the BA website (so a 125 ticket) 355 days out. It is a return to South America so LHR-MAD followed by the long haul sector.
The first problem I had is that I could not book seats at all. Then I worked out that though IB’s booking window is a bit shorter than BA’s their seating window is shorter still. IIRC I had to wait until 330 days out (maybe less than that) for seats to be visible at all.
That was all fine and dandy, but I then had a second problem, which was that I could only manage to book seats for the first three sectors, not the final MAD-LHR sector. This was using BA’s MMB function which transfers you over to the IB website. Tried a number of times over a few days and was getting nowhere. I then went into the RJ website and did it there.

PS Finally, just to add I had a third problem when (maybe an a/c change) my chosen seats on that final sector changed and I was sitting in a different row to my wife. This time the BA MMB link into IB worked. Moral of the story is that airline IT is full of quirks and you just have to be creative and keep trying.