dpkg-buildpackage --binary-version <ver> --binary-changelog 'foo' could
create debian/changelog.build with the given changelog version and
changelog entry.
dpkg-parsechangelog could be taught to read debian/changelog.build
before debian/changelog so that dpkg-parsechangelog continues to do the
right thing (when called from debian/rules).
And dh_installchangelogs can be taught to install
debian/changelog.build as
/usr/share/doc/<foo>/changelog.Debian.build-$arch.
dpkg-buildpackage would clean up debian/changelog.build if it wasn't
passed the proper option. dpkg-source would learn to not include it in
generated source packages, too.
This looks like rather appealing to me. What do you think?