GUI update-manager is able to show the "changelog" of packages to-be installed (not downloaded yet).
I also found out how to provide the .changelog files in the right place for update-manager to show them, and now I'm happy since I'm able to tell my clients that they can see changelogs of my custom packages directly from their gui.
Unfortunately I'm not able to find any command line tool to do the same thing and that would be more useful on servers.
From what I saw it seems that this convention (putting .changelog files directly alongside the .deb files in the apt repo) is a ubuntu specific extension.
There are some debian resources (the reprepro man page for example) which point on a different way to store changelogs online, http://packages.debian.org/changelogs
Does anybody know if there already exists a tool like apt-cache to show the changelogs from packages which are not yet installed (nor downloaded) ?