What does it mean that BMW does not allow LATCH borrowing? My guess is that it simply means they do not want to assume liability in case anything goes wrong since there is no evidence supporting the safety of this usage.

However, I can't see why physically, it would be any different to use the outboard ones for the center seat. It is still physically attached the same way with the LATCH, the only difference might be that it is slightly longer reach than if you used the proper LATCH points. I don't see how the longer reach would compromise the safety of the car seat as if installed properly, there is absolutely no way the car seat can move or fall off.