It's no problem. You can wear everything on the plane. I've worn almost full gear on the plane on several occasions. Hardly any questions going through security.
I usually use the helmet as a purse. I stick my gloves in there, camera, mp3 player, book, whatever - and hold it by the strap. I think this is ideal. And it fits right under your seat. I don't see the point of carrying extra helmet bag and whatever - it's just extra PIA when you need to be travelling light.
Also, keep in mind that once you're on the plane you're free to do whatever you want. You can wear your bulky jacket/backprotector/whatever getting on the plane - and it doesn't count towards your luggage weight limitations
- but once you're on the plane you can just stash all that gear in the overhead compartment.