BA confirmed there was no need for award availability for the infant - they just take 10% of the normal Avios plus a small taxes hit to add them to the booking.
Even when I tried the dummy booking with infant, the system showed only 2 CE seats available but let me proceed with 2 adults & 1 infant as the passengers (until it asked for a name and DOB for the baby). So seems no award availability required until the point the passenger actually needs a seat!