i like the idea
and there are plenty of stories where the joker is not center stage, but his present is consistently felt.  The best example of all is the longest story dc ever did in batman.  No Mans Land.
throught the whole story which was like a year long, joker was absolutely no where to be seen, but they kept teasing, saying joker has yet to make his move.  and then in the end, litterally, the complete end, he pops up to steal the show away. It was a terrific story and his scenes where magnificent, i loved when he almost killed huntress, and then when he took out sarah gordon.  
was he shown in the story before then, yes, but very briefly
before that there was knightfall,  bruce keeps saying that the most dangerouse ones like two face and joker still have not even made there move.  then when he did, it was one of the best moments in the story.
both the long halloween and dark victory had similar deals, where the joker wasnt center stage, but he was a huge presence.
see, to me, this trilogy is centered around 2 people, batman/wayne and harvey/twoface.   Its based off of the long halloween, and although there are plenty of other villains, dent is center stage, he will be the main story.
I see them kind of going with something along the lines of war games with the joker, having him setting off a huge war between the freaks and the mobsters.  Of course the freaks will win, and then batman will stop them but it will cost him dent.  I think the joker will have a role no bigger than the scarecrow in begins.  but thats fine as long as they use that time correctly. then in the end, he takes center stage for a memorable finale.  That is true joker style.
even look at hush, his role, to me, was the best part of the whole story, and it lasted only one issue. which to me would be a great scene to have, just replace elliot with rachel, and have joker really be the killer.
it doesnt matter if we know for a fact that joker is the one repsponsible, we will still stay in suspense waiting for his big moment.  
you all keep saying that for joker to be done right he needs like 80% of the screen time, and he needs to be center stage for the whole film.  WRONG
the joker is about stealing the show yes, but you cant be sure you will upstage the competition unless you have already seen all they have to offer, and make your move in the final act.