How many codepages there are? If a dozen, then you can generate empty modules for all of them that when invoked tell user exactly how they can enable those modules. So you would something like:
You are using CP1234 but we have not compiled it. Please add this to your config and then run
mix deps....
Another option is for you to dynamically generate something whenever it is missing but print a warning. That’s what we do for mime: https://github.com/elixir-plug/mime/blob/master/lib/mime/application.ex
