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

After changing the last updated timestamp for, you need to refresh the information.  The following sequence of commands works:

$ touch my_project.7.x-1.x.nb.po
$ drush l10n-update-refresh
$ drush l10n-update-status
…
my_project (7.x-1.x)   Local update available
…


Editing the embedded timestamps POT-Creation-Date and PO-Revision-Date is not required and have no effect.