I would imagine that the solution to this would be to write a service provider that has its own implementation of mergeConfigFrom that has the functionality you are seeking. Off the top of my head I can't think of anything inherent to Laravel that would do so specifically.

Thanks. So the service provider would register config file foo as a merge from config file bar, which can be anywhere presumably (not necessarily a base config file in a package). Then reading config data from foo.my_config_value would fall back to bar.my_config_value if foo hasn't defined my_config_value. Is that how it would work?

check this out http://laravel.com/docs/5.0/packages#configuration