From checking on ba.com I have noticed reward seats becoming available the day before travel but not on the day of travel but maybe people are snapping them up before I log on at around 8am. I always assumed that they do a final release of reward seats the night before OLCI opens 24 hours before the flight.

I have also seen an instance where there was no availability when I checked in the morning but a reward seat was available later that same day (the day before the flight). Perhaps due to someone cancelling their ticket within 24 hours of booking.