Can't address transatlantic but I did carry my brothers ashes (surprisingly heavy for such a small box) on a Transcontinental (MCO/SFO) 7 years ago. The only issue I had was when a TSA agent asked what was in the box and I told them "my brothers ashes, do you want me to open it up so you can look" the agent stuttered and finally said "that will not be necessary, have a nice flight
. My brother made his last flight under the seat in front of me in F/C ... just they way he would have wanted it.