Well, it took a long time and some high school math, but I did comparisons of the sizes of the empty show building in Japan's pavillion, and the layout (a buddy of mine was an architect with WDI on DCA, and gave me maps) of SOARIN's building...
Japan's show building is perfect for two omnimax theater spaces, IF they were essentially mirroring each other. In other words, where SOARIN's show building puts them side by side, in a 250 foot long building, Japan's could do them essentially facing each other, with a separating wall and entrances for each.
So yes, Japan is a good fit, though what I don't know is how tall the show building is. If anyone knows THAT, then there's your answer, but there is definitely enough square footage.
They would probably have to modify the accompanying museum space to accomodate a queue, though.