How to make Localization update believe that the translation of a local .po-file not up to date?

I use a local development branch of a contributed project on several sites. The strings that need translation does not exist on https://localize.drupal.org/ (yet), so I cannot use it to translate the project.

Checking l10n update status produces:

$ drush l10n-update-status
…
my_project (7.x-1.x)   Up to date
…

Now, how do I convince Localization update that the translation of this file is not up to date?

So far I’ve tried:

• Editing and saving (i.e. changing the last updated timestamp).
• Editing the embedded timestamps POT-Creation-Date and PO-Revision-Date.
• Clearing the cache.

But no joy. Localization update keeps insisting that translations from this file is up to date.