FredT once bubbled...
If I remember correctly EU tanks have test pressure stamped right on them, so any hydro shop that can match the valve thread should be able to hydro them, they just can't do a DOT approved stamp on them. Without the DOT stamp most shops won't fill them so you're still out in the woods.
US bottles are tested at 9/5 of the working pressure but European tanks are tested at 3/2, so I don't think the hydo thing will fly. That is why the Scubapro 95's in this country are 110's in Europe, they test differently and fill to a higher percentage of TP.