You're barking under the wrong tree. I never said that FFXIII would have sold 1M PS3s if exclusive. I talked of something about a few hundred thousands, which by itself is about 15% of the 360 sales. The remaining 60something% (around 950K sales in the west) would basically come from the overlap, ie owners of both consoles.
As you yourself reminded, MS seemed to have a definite strategy for JRPGs once upon a time (lost odyssey, tales of vesperia etc.), leading - I think - to a substantial niche adoption from JRPG fans who wanted to encourage this policy for the west. Look at Lost Odyssey's sales outside japan: about 800K.
Just for fun, you might start a poll on the forums to find people who bought FFXIII on the 360 to find out if they also own a PS3 or what they would have done in case of exclusivity. These forums are demographically extremely different from mainstream, but maybe not that different from the niche JRPGs audience.
