I'm not too familiar with Expositiones Mathematicae, but have you given them a look?

EDIT: The article I happened to have seen, which made me think that Expo Math might be along the lines Pete Clark was looking for, is this paper of T. Bühler - it modestly claims to no originality save for assembling disparate parts of the literature and writing down what's old news to connoisseurs (I'm paraphrasing here!) but of course this is, in a sense, precisely its originality & worth.

I'm not too familiar with Expositiones Mathematicae, but have you given them a look?