Just referencing modern day and what people would know it from. I’ve used factories way back since before C was standardized so I know it is very old pattern. ^.^;
This is the big key!!
You can put the ‘hardcoded’ map in the config files, then in the app anytime you need it access it via Application.get_env/2 and query it directly. You can even do that at compile-time to build up function matchers but by keeping it at run-time you can allow for it to be changed while the system is running to add/remove things to it without needing a hot code swap or reload. 
The configuration itself would just be a mapping of stringnames ↔ module names, or even full MFA’s if you want.
Protocols are for dispatching on a type like a struct type so if you have that then absolutely yes, otherwise the plugin model is better (which you can also do via any arbitrary tag, like a string, with my ProtocolEx library ^.^).
