The fact that the CaLV is being made man-rated too, would seem to indicate that the CEV will be able to fly on any LV which is capable of a) lofting the mass to an appropriate insertion point, and b) doing so while conforming to the safety requirements.
NASA is only going to pay for one single Crew launcher though. If congress decides it needs a backup CLV, they can direct NASA to develop a second one. They haven't, and I suspect they won't.
So that leaves another organisation (private or federal) to fund one. There's no reason why another can't be developed courtesy of a different funding source. And there's still no particular reason why a foreign agency couldn't approach the US for CEV's assuming they have an LV capable of lofting it. There's a lot of red-tape involved of course, but it would be possible - assuming the right money.
Ross.