Interestingly, last night i was able to change a MAD-MEX booking for my son and I (made with a 2-4-1 voucher through the BA website) so i could end up on the same flight as my wife and daughter (who also booked onto that flight with a BA 2-4-1). All of us in business.

Not sure if Iberia give the extra availability through the BA 2-4-1 - but it's the first time i've ever seen more than 2 seats available for a business class avios redemption on IB.