Can we add hook_locale_alter ?

The use case is the following desired behaviour:
Where a localisation string is not found, rather than just returning the source code English string, return the translation in a nominated other language (which is assumed to be more fully localised). This is required in minority language situations where lack of resources or humanpower might mean that localisation has not been completed, but resorting to English is culturally or politically unacceptable.

The desired behaviour will never make it into core, but in order to implement it in contrib I need a hook_locale_alter (or some other name!) which allows a module to implement a different database query (or different logic) at the point where the query is made in locale(). I'm assuming this can be done efficiently so that we find out once whether such a hook is defined, remember that and, if so, call that hook after (or instead of?) the existing DB query, i.e. only where the outcome is not already stored in cache. The overhead for any site not implementing the hook would therefore be one single call to module_implements - can we afford that?