The US based freighter is a junior position. That said it is currently being phased out over the next few years as the payscale is now unified and by 2011 there will only be SO and FO's.
Most of the expansion of the freighter fleet is into the US in the future, in other words you will have no problem getting back to the US at sometime and as the pax fleet is becoming 777 then your upgrade from SO to FO can be on that fleet and you are sweet!