Hey, Lenore -- probably depends what kind of "croute" (that was a typo, right?) you're preparing. A puff-pastry type? Particularly bad at being re-heated. A pie-crust type? Not so bad at being re-heated I've found.
But then there's the brie in the middle which probably will, like most things, be at its very best freshly heated.
As for covering it so that it doesn't dry out, won't plastic wrap do?
Good luck -- sounds yummy!