Here's my suggestion, borrow a rear facing baby seat from a friend/neighbor.
Place the seat in the back, behind the driver’s seat, load up your SO and see if he/she can "tend" to the pretend baby in the back seat (by running her seat all the way back, and tilting the back of the seat all the way back).
Then for sole runs, put the make believe baby in the passenger side rear seat, and see how you do ..... tending to the baby.
When they get older (up to 8 years old) they (up to two of them) will "fit" OK in the back seat ...... for short trips.
Note: the limitation with the FR-S/BRZ might be all the crap people with babies haul around ......
humfrz