Well, I also started to think of an alternative approach: For contrib plugins, AMOS could automatically include the translation in all versions higher than the "real" once if the plugin doe snot have explicit strings registered there.
So for example, if there is a contributed plugin mod_foobar registered in AMOS on version 2.1 and you have it translated into your language, the foobar.php would be automatically present in 2.1, 2.2 and 2.3 ZIP files. This seems to mimic the legacy behaviour well for contrib plugins.
I just can't tell yet if this would have an performance impacts. Generating ZIP packages is already pretty time consuming job (takes approximately 15 minutes for each branch) so we might need to return to the original schedule and generated ZIP packages once a day only or so (instead of every 3 hours as we do now).
As for the performance: how long do you think to keep the old (read unsupported) branches for David?
I'm really not sure Koen. For example, the support for 2.1 is going to end soon and will be switched to the security-fixes only mode. I guess it would be enough to generate strings for such branches just once a week or so.