I've been monitoring this particular BA route for the last 18 months or so. In short, BA is often loading very close to the minimum at the 350 day mark in CW. That is one flight per week. I think it was Friday's out and Tuesday's back being the most likely, though some weeks this varied and extra days added in.

The work-around I used was to look at GRU too. I decided the CW experience and then a swap onto LAN or AR metal at GRU was bearable. This has worked out for me as GRU is fractionally more available, I was even within a day of the perfect dates!
However, the Iberia option is not that far behind in my book. The problems for me were it needs to be a full mileage reward not an MFU and the return timings lose me a business day.

Once less than 300 days out I've rarely seen more than the odd lonely seat pop up.