Thanks for the additional comments. I realize I might have to explore carseat.org but I thought I'd try tapping the collective wisdom here one more time

Does anyone know of any such products that accommodate the smaller end of the toddler range? My kid will be around 2.5 years old, and I'm guessing a little over 30 lbs when we will need a travel car seat. The Boostapak recommends a weight of at least 33 lbs (so OK there), but an age minimum of 4, so a little under there. Still, it might have to do if I can't find something similar that's better suited to younger kids.

Clearly the portability of the Boostapak can't be beat... I love the idea of going out for the day to explore a city and having the car seat in a backpack I carry in case we need to take a cab somewhere.

If lack of FAA approval is a problem when it comes to Boostapak and aircraft, then I'd probably just get a CARES harness for use when on board.