I did a similar thing in Abu Dhabi. I had just arrived and my first task was to to carry out an air test in a WS55 Whirlwind. I got airborne and left the zone to do the air test outside controlled airspace. When I was ready to return, I couldn’t remember the aircraft registration (callsign) and there was no placard in the cockpit. I though about the problem for a couple of minutes and decided to make up a false callsign. When asked by ATC, “What happened to VR-XXX”, I said “it went u/s”. No further questions asked. I haven’t a clue who got the landing fee charge.