Just to clear up any possible confusion... there is no event in the game programmed to allow you to battle any old Champion in the game aside from Cynthia. Events like the Darkrai event are programmed into the games from the start; they're just not unlocked until you receive the pre-determined tickets/items/event activators. Not to mention there isn't any trainer data or sprites for them, either. So, it's impossible to battle Red, Green, Lance, etc. in Black and White.
That being said, it's possible the Champion Remix OST may be used in the future; I'm specifically referring to the United Tower, or the third version. Or, it can simply go on unused. It's a good remix; I wouldn't mind if it went unused, but it'd be a waste then.
As for myself, I would've loved to battle an old Champion; but we just got the opportunity to battle Red, Green, and Lance all in HGSS, and I'm pretty content with the cameos we have now (although, I wish Cynthia was battleable every day, but oh well).