I recall that on BA 737's for European travel we could only select seats in certain rows due to the requirement for extra oxygen masks. This doesn't seem to be the case on QF, we have been seated in rows, 5,6,7,9 and more over the years.
The exception is that some of the QF 737's have airbag seat belts fitted in row 4, and infants are not allowed in those seats. For that reason, you cannot select row 4 with an infant in the booking on any 737 IME. You can however select with a child in the booking, but you may find that you are moved to another seat if the aircraft has the airbag belts in row 4.