Any Unicode encoding would allow that. UTF-32 has an entire spare byte to put flags into. 24 or 21 bit encodings have spare bits that could act as flags. UTF

Any Unicode encoding would allow that. UTF-32 has an entire spare byte to put flags into. 24 or 21 bit encodings have spare bits that could act as flags. UTF-16 has plenty of invalid code units, or you could use a high surrogate in the last 2 bytes as your flag.