*tosses a hat in as a potential Gm, pending some discussion.*
As far as blending in goes, I think that there are a few options available on that front. I won't have the chance to check my own books until I get home from work this evening, but if there aren't any long-lasting disguise spells, then perhaps a custom ritual might work? Alternately, there is a feat in the document for dragons to get a humanoid vassal, as well. I suppose it might get a bit hectic if everyone is essentially playing two characters at once, though.
Depending on the world the dragons get sent into, it might also be a simple matter of letting the metallic dragons in the party act as the faces of the group. If it's a world where dragon alignments follow the usual D&D color-coding, then there probably won't be as much screaming if the people are approached by the dragon breeds that they're used to thinking of as noble and good.
As for the overarching plot, getting sent out to deal with some renegade dragon or new major threat could certainly work. Maybe there's some vast evil horde that's amassing, or an army of giants preparing to go out and conquer the world. Alternately, though, perhaps the players could be the ones who are the renegade dragons? Banding together and striking out on their own, away from the flock, to carve out their own little niche in the world?
And Muse, considering how few other players have shown their interest at the moment, I don't think their will be any objections to having another player join in. ^^