Well adding it to allowed character doesn't work, so scratch that.
And for the handler you are probably already at that point. I suggest taking this to code and use .NET's String.Normalize as mentioned here: stackoverflow.com/.../how-do-i-remove-diacritics-accents...
In case you really need it to be done in a macro you could create your own custom macro to handle it:
docs.kentico.com/.../registering-custom-macro-methods