Where in strict baggage handling "rules" does it say that you can be allowed this? As that'll be what you come back to when security says No No, you've already finished your check in, and now your stuck with a $300 extra suitcase fee?

I've taken several large bicycle parts as carry on bags (even a small frame), but I assure you'll get hit one of these days. I had a just larger than official carry on size case flying BA from Beijing and I got stopped and forced to pay to check it there. It's clear this isn't a baby stroller or a wheelchair. Why wouldn't they gate check my little bit big carry on? And why would they do that for the Brompton?

I'm happy that you can do this - but it's the same situation like hoping to fly with a classical guitar in a softpack. They may tell you No No and then you have nothing to check it in, and chances are there isn't enough time since the flight has been closed to go back to the gate.

pps - why not just remove the seatpost when running the Brompton through security? One I've got with the telescoping post makes this easy , 'know that the stock model as the flanged area on bottom to keep it from coming out.