Its more of a "we'll see" kind of thing as it is, so nobody should take this super seriously.
In response to certain people saying that it would be impossible, however, solely because of the fact that Japan would lose out on that content, there have been games in the past that included extra content within the American and Pal releases that went beyond the original Japan release.
Two examples that come to my mind are Final Fantasy 7 and Kingdom Hearts. In FF7, most of the optional Weapon bosses near the end of the game were included in the American/Pal versions, while they were absent in the Japanese release. Same thing with Kingdom Hearts, there were optional bosses in versions released after Japan's. However, in those cases Japan did eventually get the content, and in Japan they even released Kingdom Hearts games with even more content than the American version.
That's not to say that Smash is a totally comparable situation, I mean its two different companies thinking differently. And the way Japan would get extra content is through downloads rather than a rerelease, so don't start flaming about "why would they rerelease brawl 2 months after it was released in the first place?" or anything like that.
What interests me the most, should this happen to be true, is that if Japan DOES get Roy and Mewtwo through DLC, would that open the doors for more DLC for Brawl in all regions? Or would it just be some kind of system update that would "fill in the gaps" as it were?