Summary of what was written in the other related question and in comments:
- You may put any 'printable' ASCII char in your custom header value field.
- If you want to use special characters, encode these characters following whatever rules/charset/encoding you choose. As long as this encoding it uses simple ASCII chars, it's OK. An example is using UTF-8 and encoding string chars to \u%%.
- On the server side - you must manually make sense out of the encoded string, probably by decoding it using the rules of the character set/encoding paradigm you chose before.