How to use std::error_category with external messages?

If I understand how std::error_category works, it's meant to be a singleton that holds hardcoded messages to be returned when a specific std::error_code calls the message() method, internally using that error code to identify which message to return.

My question is how can I use the pair std::error_code/std::error_category when the messages I want to return are created dynamically and cannot be cached?

Following this tutorial I'm stuck at overloading make_error_code so I can pass a string to my child of std::error_category. Is there an alternative?