Yes,SeatSpy etc is a good place to get an idea for single flight availabilty. You can see where there is J and if you keep an eye on it, how quickly it usually goes.
But you wlll probably need to search on BA as SeatSpy a) Doesn't include Amex voucher availability b) I don't think it does availability for journeys with a stopover.
The availability is not quite the same as the availability of seat on each of the individual legs (can be better). Maybe someone who knows more might comment on that.