I would try to configure my code editor so that it stores files in ISO-8859-7 encoding and it should happen "automagically". You just type the characters in strings and that's about it. When you write "[ch916]" it will store 0xC4 in the source file, just like it stores 0x61 when you write 'a' etc...
EDIT: That is: in your example, the "c" variable should already contain the correct code, exactly as it behaves for lower part of the ASCII table.
But you must be careful with those files, if you edit them with some other editor (encoding) it will probably corrupt the strings.
Or you could escape greek characters as codes in strings: like
"this is delta \xc4 isn't it?"