No one seems to have mentioned an important factor. There are dozens of US registered aircraft operating within Europe, and whilst they are still allowed to come in, European operators will never be able to establish themselves so that they can satisfy the needs of DHL, BA etc.
If DETR say "OK", then AFX, Channex, Heavylift, Air Foyle, and others will have no incentive whatsoever to acquire an aircraft that will do the job, bearing in mind that there is an economic hurdle to overcome right at the start, ie US operators are inherently more cost effective than European operators.
But with better economy of scale, this hurdle could be reduced very considerably.
OneWorld 22, I accept your info, but I am sure that wet-lease operations are not allowed within the USA, so there must have been some sort of exceptional circumstance at play. I'll go along with ShotOne.
Ho Lee