Thanks to Mehdi Dogguy, here's a nice hook to generate a source change file at build time (with pbuilder), so one can upload source-only packages to the archive and have buildds rebuild for all the architectures. Put it in .pbuilder/hooks/B10_source-build so it gets called once the builds succeeds

#! /bin/sh

generate_change_file()
{
  local version=$(dpkg-parsechangelog -Sversion)
  local package=$(dpkg-parsechangelog -Ssource)
  echo "Generating source changes file"
  dpkg-genchanges -S > ../${package}_${version}_source.changes
}

cd /tmp/buildd/*/debian/..
generate_change_file

Next time you build a package, you should find, alongside the <package>_<version>_<arch>.changes file, a <package>_<version>_source.changes which you can use with usual tools (lintian, debsign, dput…) to upload it to the Debian archive.

Note that if you do that, you have to make sure that your debian/rules support building separately the arch-dependent and arch-independant packages. To check that, you can call pdebuild like this:

pdebuild --debbuildopts -A # binary-only build, limited to arch-independant packages
pdebuild --debbuildopts -B # binary-only build, limited to arch-dependant packages