Maybe so, but DMC4 was only a late entry, it was strictly PS2 up to that point. Sony has no real say anyways, since it is Square-Enix's property. They wouldn't want to anger them and miss the opportunity for it to be released on their system as that would be quite a lot of profit to lose.
If they did it in this generation? Of course they'll do it on 360 too. At least in the U.S. I mean... Honestly, its easier to port a ps2 game onto the xbox 360, than it is on the ps3. Just for the fact that, the cell and graphics chip is nothing like the ps2 had. Whereas the 360 has essentially, normal pc parts inside of it.
There wasn't any reason to do it on the original xbox, for the install base it already had. But...Considering how long its taking to release final fantasy 10 hd? I doubt they would do the same with kingdom hearts anytime soon.
MGS HD got ported to 360, maybe only being that MGS2 was on xbox and MGR is going on 360 (they get raiden's story?) you got KH 1 2 3, RE Chain of memories and birth by sleep,could be in the HD (since peace walker was in the HD with online and KH BBS HAS MULTIPLAYER like peace walker.) its possible but i doubt it, to much licensing and if I recall birth by sleep never even got its digital release due to licensing. but it would be sweet.
There's only one reason I can see that can keep a Kingdom Hearts remaster off the 360, and it has to do with who owns the characters from Final Fantasy VII. Unlike most Final Fantasies on Sony consoles, Sony Computer Entertainment actually published VII and VIII. If they own the rights to the characters then you can bet that Kingdom Hearts won't go multiplatform. But, if Square-Enix is the sole owner, there's no reason why KH can't go multiplatform.