I think this should be relatively straightforward once we have bug 1397552: we can then trigger the nightly-l10n, based off the previous graph's build/build-signing/repackage/repackage-signing of en-US. The downstream tasks (beetmover, balrog, funsize, etc) would then need to depend on the nightly-l10n kind. We would have to make sure the mozconfigs + mozharness configs we point to are beta / release oriented. That means either pointing at different mozconfigs + mozharness configs, or patching the nightly ones on merge day.

In theory, this will allow us to add l10n tasks to the promotion graph. This patch depends on bug 1397552 to trigger the promotion graph dependent on the 1st (on-push beta) build graph.

The above patch is landed on date-branch, and other than a typo fix, appears to work well.

We're using tc l10n in fx + devedition + fennec betas. This will ride the trains.