First point to consider - that seam between the hull halves will only be visible, under normal circumstances, for a few inches. Along the bottom of the keel it won't be seen unless somebody picks up the model and looks at the bottom of it; at the stern it will be hidden by the rudder (though the latter will have a similar seam in it - and isn't pre-painted). The only visible section of the seam will be at the bow.
I don't know of any reason why any sort of repainting wouldn't work. Beware the use of the term "leaf," though. Actual metal leaf (gold, silver, copper, or otherwise) is, by definition, only found in sheets. (The term "liquid leaf" is an oxymoron. At least one company uses that term to describe its products, which are actually just fine-grained metallic paint.) Honest-to-goodness metal leaf requires some sort of "sizing" underneath, to serve as an adhesive. I've tried using gold leaf on models several times, and frankly I haven't been especially impressed with the results. In my experience it doesn't actually look any better than any of several excellent gold paints on the hobby market. (Testor, Humbrol, and Floquil all make metallic colors that are at least as fine-grained and shiny as anything the art supply stores sell.) I've never used copper leaf, but I'd be concerned for the same reason. Also, I'd be worried that the coat of sizing underneath the finished coat might obscure some of the excellent detail molded into the plastic hull halves.
Then, of course, there's the question of what color the stuff actually ought to be. We've taken that up in several Forum threads recently; do a search on the word "copper" or "sheathing" and you'll find a couple of dozen interesting posts. There's plenty of room for interpretation and taste there. The discussion starts with the fact that the sheathing on the Cutty Sark is not copper; it's Munz metal. Then there's the question of just how that material actually would change color - if at all. One thing we can be sure of: air won't have the same effect on it as saltwater.
Maybe copper leaf would be worth a try; if you do it, by all means let the rest of us know how it works out. But I wouldn't let the problem of the seam between the hull halves be the driving force in your decision. It wouldn't be difficult to fill that seam and paint it; just confine the new paint to the row of "plates" on the very edge. Even if the newly-applied copper paint doesn't quite match the pre-painted hull, the difference will hardly be noticeable.
Youth, talent, hard work, and enthusiasm are no match for old age and treachery.