Maybe if she is very tiny?? Being that you're Australian, this is very unlikely

Get the Scenera for this trip and then whenever you make this journey again, remember that the highest slots are 15". Before you leave, have her sit against a wall and measure how high her shoulders are and then you'll know whether she'll still fit in it. Most kids outgrow seats by height before weight. I had another seat with shoulder straps only one inch lower and she outgrew is a whole six months earlier.

If the Radian65 were cheaper, that would be perfect. Rear facing to 35lbs, forward facing to 65, my 5 year old is only now in at the highest harness slot. It would only be worth it if you were taking it back with you.

FYI, all American cars made since 2001 have to have tethers.

By about age 4, you can look into the Saferider vest, which can't be used on airplanes but you could easily take back and use in the car. Great for travel, especially taxis. Then again, who knows what they'll have on the market in a couple of years...