You could try supporting a subset of languages to fit in your requirements. For instance disallowing tab characters and using them for control.
Additionally problem character replacement isn't so bad if it is machine controlled, since it can be more complex and making it human editable isn't a problem since your current format isn't human editable.
Additionally problem character replacement isn't so bad if it is machine controlled, since it can be more complex and making it human editable isn't a problem since your current format isn't human editable.