I'm back from Japan, and the conclusion is - the H-type does fit through the security scanner. Success! Attached image is of the bikes at the gate on our first flight.

It wasn't entirely smooth sailing. The long story:

• At SFO - check-in and security: I was pretty nervous, this being the first time I tried flying with the Brompton. Of course we were unable to check in online, our reservation could not be found in the United system, and had to stand in line to talk to United folks in rising order of competence.. I felt a bit cagey, hiding our luggage cart behind us and hoping they wouldn't ask. They didn't, just accepted that we wouldn't be checking any bags. At the entry to the security check I thought we were in trouble - they pulled over my wife and I thought "uh oh!", but it turned out to be because she had 3 items and not because one was a bike. The people at the actual x-ray scanner were very nice, lots of friendly questions about the bike.
• At SFO - at the gate: based on threads here, I felt we should go straight up to the counter and ask to gate check the bikes so as to avoid the gate folks having to deal with it when they were stressed out during boarding. The first person we dealt with was super nice, checking the bikes through to Narita and giving us priority tags. Then a second person showed up and really grilled me on how I shouldn't have brought the bikes to the gate - should have checked them in and paid a bike fee. He really went off about how bikes are different from wheelchairs and strollers, and claiming it was a "mobility assist device" was an abuse of disability law, bla bla bla.. I told him there was no reason I should pay any fees, because the bikes are smaller than the maximum luggage size (and in fact smaller than the carry on bags many other passengers had!) and weigh less than our allowance. That shut him up, but he called his supervisor. The supervisor showed up, and was initially a bit icy - but after a call to the supervisor's supervisor, he warmed up and everything was ok. No fees, no damage waiver.
• At Tokyo Narita: Malaysian Air would not accept that we wanted to gate check the bikes, and insisted we check them. But no talk of silly fees, and they even provided bubble wrap and tape for us to package the bikes. Had to sign a damage waiver for "missing or inadequate packing".
• At LAX:We had to pick up the bikes to go through customs at LAX, and walk them over to the United terminal. Again we just told them we'd gate check the bikes, no problem. Security was a breeze (and I wasn't nervous this time!) - though security guards in LA are apparently not cyclists themselves, like in SF. At the gate we decided our new strategy was to wait until boarding, so the staff wouldn't have time to make problems. However, the flight was going to be full and they asked people with larger bags to come forward and gate check - so we did. No issues about bikes, but they refused to give us priority tags or even fragile stickers this time.

Results/Conclusion: Flying with the Brompton is pretty hassle free. Once we were in Japan it was a pleasure to have our bikes there, and the Bromptons were way less hassle to travel with and bring on the train than our friend who brought a Bike Friday and towed the suitcase as a trailer (though obviously if we were doing self supported touring with camping, or just day rides, then the BF probably would have had the edge). My bike got a couple of scratches through to the metal, but otherwise no damage. I'll get some touch up paint and all will be well..