In the past I have loaded my backplate and harness into a backpack, then stuff 2 regs into my reg bag, which fits in the same backpack. Then my mask, dive computer, logbook, ipod, etc... The reason I carry on the BP is for the weight issue (stainless plate) to free up more weight in the checked bag. I have since switched to an alum plate, and got the Dive Caddy. I dont see me using it like its intended for, mainly as a better backpack. The old one is a bit uncomfortable.
Never had an issue with the plate thru security, but I always pull the reg bag out of the backpack as it usuially get the wave-around to the hand search. Not an issue...I go thru the line farther ahead of my wife and it usually times out right - I get done and she goes right thru.