Ok, i have a friend playing in us and i am in eu and i tried to play with him over the us realms on ptr(since eu servers are also located in us).
First i tried changing the realm id with google chrome (swap brill with broxigar) but the damn thing won't bite.
A few days ago i saw on eu forums ppl whining about their copy page forcing them to copy us chars and that their live copy chars is locked(it has been fixed but it gave me the mr obvious idea that i should have seen it myself as a chance).
What you must do:
1. Log in as u wanted to copy your premades on eu page.(i tried to log from us page and it seems that it will reject me)
2. After you are logged it change the link from wow-europe.com to worldofwarcraft.com
3. Copy premades to us realms.
I haven't got the chance to test the other way around but it should be the same