That is a specific Diamond benefit. In fact it was one of the reasons given me by CX when I asked why I would want to be Diamond when I am already Emerald. It wasn't made clear what conditions allow releasing the seat.
I guess I fly the wrong flights on the wrong dates. Maybe month/quarter end has something to do with it. I have lots of trouble finding seats in A, but I usually have 3 or 4 flights to co-ordinate, so if one is not available the others can't be used. In 2003 there was no problem with these bookings, in 2004 it takes multiple attempts and rerouting, so the availability is way down from my standpoint. Bad enough that my last AONE4 had only 2 A sectors, the rest were D. I guess I have to spend more time finding the flights, or have less tight of a schedule.
BTW, that 3500 figure is very disingenious (just noticed you did qualify it as last year -- it wasn't a problem last year, it seems like a problem this year!). It might be true, but excludes all the contracted upgrades (buy J, fly F -- lots of companies now have that deal, hence the "high loads in F" albeit at J revenue ... VS has had a big effect with their new suites in Upper Class and this is BA's counter attack). Feel rather foolish actually paying for an F ticket on BA these days, nobody else does.