So let me see if I understood this correctly - you want to be able to add new phrasegroups/actiontemplates to existing vB pages?
My suggestion:
AdminCP:
New file: Import/Export/Editing/Deletion of an XML file record containing list of templates/phrases required for each mod. A dupe of the plugin system, so to speak, only it contains a possiblity to amend to every single instance of the $xxxtemplates arrays, including subarrays.
Every time something is added/edited/removed from this list, a flatfile (similar to the flatfile DS cache option in vB) is created. Obviously it does array searches to make sure no duplicates are present (although they wouldn't do much harm).
Init.php:
Find
PHP Code:
require_once(CWD . '/includes/class_core.php');
Add below
PHP Code:
require_once(CWD . '/includes/datastore_precache.php');
From there, different parts of init.php can use variables from this file (again, similar to the datastore_cache.php) to do whatever it is you want it to do, such as array_merge($precache_templates, $specialtemplates); etc.
A simple require_once() won't bring any huge load onto the server, it won't be parsing anything and it won't be querying anything.