FWIW, I had this same issue even on ticketed BA business award flight on IB metal (MIA-MAD-MIA). Everything booked originally via BA website. At time of booking, BA even provided the IB PNR locator (which was some odd 5-digit code that didn't seem like a true PNR). However, when I went to the IB website I could access my reservation in IB's system, but I still could not select seats. I had to call IB in the USA to request them to manually choose seats for me. Thought this was quite odd given the nature of the BA/IB relationship, same parent company. Perhaps IB is locking out all code-shared, non-IB ticketed award bookings from being altered online (even on IB's website with IB PNR). I've searched IB's forum, but haven't had any luck with this one. Good luck and keep us posted!