I wanted to share my experience with stuffing my 3 kids in my BRZ. I daily the BRZ, which in happier times means I drive all 3 to school and the older 2 to after school activities.
My kids are 8, 6 and 2, all are fairly average in size (around 50th percentile in height and weight). I'm 5' 11" and weigh 140 lbs, so I have the front seat a little forward, leaving some leg room behind me (I can even squeeze in behind myself).
My child seat setup is like this - a booster seat behind the driver, a harness seat behind the passenger and a backless booster in the passenger seat. Specifically a Graco Affix, a Britax Boulevard ClickTight and a Cosco Rise.
The Affix and the Boulevard (secured with the seatbelt) fit very well in the back seats, but the front backless booster is a challenge to fit properly - a backless Graco Turbobooster was too wide, pushing on the seatbelt buckle and making in hard to buckle and unbuckle the seatbelt (even for an adult). The Cosco Rise is much narrower, but the shape of the "hand rests/belt guides" interferes with the passenger seat bolsters a little, so it doesn't fit all that well. I'm not happy with either and plan to get a better fitting backless booster.
The kids all get in through the passenger door (since the front seats don't return to the same position when folded and I don't want to keep adjusting the driver seat), with the younger kids scrambling on their own into the back. With the passenger seat folded and all the way forward there's plenty of room for me to buckle them up. Then the passenger seat gets pulled back and the oldest gets in the front.
Rear leg room behind me is enough for the kids when they have regular shoes. Winter boots don't fit, though, so they take them off. Measuring gives around 6 inches behind me:
Safety-wise the biggest concern is placing a child in the front passenger seat. Most organization recommend a minimal age of 13, and this is aimed mainly at proper position with the seatbelt and for the airbag. Digging a little deeper, the airbag is the main problem - it is designed to protect an adult occupant, and might injure a child when deployed, so the recommendation if you have to have a child in the front passenger seat is to disable the airbag and have the seat as far backwards as possible. With the capacitive occupancy sensor in the BRZ it is impossible to disable it, but I found that when my oldest sits on the backless booster the airbag is disabled (which is what I expected). Thanks to the youngest (and smallest) kid being behind the passenger seat, I can move it almost all the way back.
Finally, states (and countries) vary by law about this. In the US, most states allow younger children in the front seat if all other seats are occupied. Ohio is like that, but your state might not be.
At the end, everyone takes their own level of risk with themselves and with their children. I am on the stricter side in general (my older kids were rear facing until 4 years old, and the youngest is still rear facing in the Odyssey), but I feel that the compromise with the BRZ is good enough - the kids ride in the BRZ only for short trips (with all trips that include my wife are in the Odyssey) and almost exclusively in the city at lower speeds.