Perhaps you could let users supply a functor open_file(filename), and they can choose to open the path however they want, from whatever source. The default functor would use what you currently have. Would this be prohibitively difficult?
Yes, it is doable. However I don't think it is reasonable, I hadn't seen any localization system that does this. Putting dictionaries in file system and using them is something common, also it does not have any performance impacts as dictionaries should be loaded only once into the memory, unlike other game resources like video or images where fragmentation or access time may have effect. In any case, I would say, you can always re-implement your own boost::locale::message_format facet to deal with such non-standard situation. Artyom