One for the future perhaps, but for a few pounds/dollars I think it was you can set up a seat alert on ExpertFlyer (without full membership) to be notified as soon as specific blocked seats become available. I did this recently for bassinet seats in Y and they became available at just over T-72. Although the seats were available to be selected in BA MMB when trying to confirm I got the message that they were only available for passengers with infants. However when selected on rj,com MMB the seat selection stuck. As already noted though there is a risk of being turfed out if the bassinet is required. Thankfully in my case it wasn't and I got a decent bulkhead window seat.