In my Country , and for domestic flights, when you make a flight plan you can write what you want in field 7 as long as it does not exceed 7 characters and if you put in Field 18 REG and your ICAO A/c registration.
If you write DICK1 you will be called/answered as Dick1 , .
Wether this is ICAO kosher or not I cannnot say but it works and very often done during competitions or by the military.