Well my immediate guess would be that Legolas will just show up in some of the fight scenes and never be mentioned. Tolkien has never been big on describing the fight scenes, with the details glossed over, or the heroes coming accross the aftermath of things like Gandalf throwing down with the Ring Wraiths. In fact, if you take Tolkien's descriptions accuratly where they apply, it actually sounded a lot like Gandalf was grappling with The Balrog literally... which I suppose is possible given that he's an Istari and a being of equal supernatural power, but given the portrayal of him as a little old man that occasionally brings some odd images to mind when I read it.
At any rate Peter Jackson is big on developing the fight scenes, and that will probably include the whole bit where the Dwarves eventually get themselves captured by the elves.... understand in The Hobbit these Dwarves get captured and need to be rescued by pretty much everything, trolls, spiders, goblins, elves... but despite all of these they are supposed to be some truely vicious warriors and heroes. Bringing out Legolas might be a way of sort of justifying how it happened....
Though to be honest Peter Jackson has never done much justice to dwarves, using Gimli as pretty much a comedy relief character, it might have as much to do with the chosen actor and the difficulty of finding a stunt double, but one thing to consider is that in the books Gimli is supposed to be just as kick ass as Legolas in his own way, indeed when they are fighting at Helms Deep they are actually having a contest to see who can kill more Orcs, and I seem to remember their personal kill ratios get up to around a hundred apiece (it's been a while) Gimli's comments about being slowed down because "one orc was wearing an iron collar" isn't supposed to be impotent bluster/humor, but a literal truth, the guy is pretty much the prototype for "Dwarf fighter as axe wielding death machine" of D&D fame.
So basically it could be done, Tolkien was vague enough with some of the details in The Hobbit where he could have been there, and going by what he wrote later it sort of does make sense that he would have been.
He could also be present during "The Battle Of Five armies"... though personally the guy I'm hoping actually gets a bit is Beorn. Basically a barbarian that turns into a giant semi-invulnerable bear who gets mentioned very briefly. Tolkien's writing when I read it reminded me a bit of like a norse Saga where the list of honors as to who was present in the battle trumps the actual action, though in some cases there might be sagas dedicated to each individual hero that explain what they did to warrent mention seperate from the original.
There is an old joke from when I was in school where we were looking at some REALLY bad stuff from Norse mythology and such (in my opinion) about how we'd have some great warrior king making breakfast with each implement and dish getting it's own line, and then each having it's own poem explaining things like how he came into possesion of the epic butter knife of Yasala and how it once buttered a hundred muffins in a single breakfast!