Hmmm... I'm thinking I might do something similar. I'm heading out for a business trip soon, and plan to bring the Brompton. I might pack my saddle into my carry on since it's relatively dense but not extraordinarily bulky (compared to the helmet). I can put a few more things into the B&W case then (e.g., liquids).

Scooper, do you bother providing any extra protection, such as wrapping up the body hinges, or taping up the fenders so they don't bend if the bike shifts in its case? Thanks.

I've thought about it, but haven't bothered. I'd guess I've made a dozen trips on Southwest Airlines with the bike as checked baggage and it hasn't been damaged.

With the bike folded, one dimension is a couple of inches shorter than the other, so an extra strip of high density foam rubber to fill that void wouldn't hurt. The thickness of the folded bike keeps pressure on it when the case is closed, so that prevents it from shifting wildly inside the case as it's handled.