Hi,
We've lived in 5 countries in the last 6 years. Needless to say, we travel a LOT. While checking your seat may not be ideal, sometimes it's necessary. Here's a few things I learned:
If you use a bag (we have 3 kinds for our 2 Britax Marathons), they are all pretty much junk. I have often used heavy duty trash bags to the same effect just ripping a small hole in the bag where I want them to attach the flight check strap. If you use a professional bag, you can stuff lots of junk in there. My favs are coats, blankets, extra shoes, whatever. I usually put them in a plastic bag, tie it up and "buckle it in". That way if anything happens to the other bag, my stuff makes it clean and in one piece.
There are some devices to help you get through the airport with your seat AND your hands free. One is the Monkey Pac [ame="http://www.amazon.com/Cheeky-Monkey-Designs-9277-Back/dp/B000GIBYQ and the other is the Toddler Travelling Accessory http://www.amazon.com/Traveling-Toddler-Seat-Travel-Accessory/dp/B000JHN3AS/ref=sr_1_1?s=baby-products&ie=UTF8&qid=1317150827&sr=1-1"]404 Looking for Something?[/ame]. Britax tends to get heavy after a half mile hike through the airport so I like the rollar board option myself.
Putting your seat on the plane is ideal but with a big kid and a big Britax, I'd be suprised if they both fit. Not sure how long your ride is, but after my son was about 2-3 it was torture to put him in his seat cause there is NO leg room. Usually the "thoughtful" people in front of him would recline and pinch whatever room was left and pinch his legs which you can't undo. I would actually sit there with my hand on my seat and keep them from hurting him and explain. On a long-haul flight, this makes for grumpy kids and grumpier neighbors! Plus, if the flight has individual screens or those kind that flip up out of the armrest, the kid is at an angle where they can't use them which is no fun to explain to a bigger kid either.
That's my travel 2 cents. I hope some of it helps!