An FAA ATP with 20,000 hours will have no privileges for flying a spamcan in EU airspace (N-reg or G-reg).
He will have to get an EASA PPL, for which there is proposed to be some sort of conversion route (exams and a flight test).
Of course it is barmy.
Currently, the UK CAA automatically validates ICAO PPLs and CPLs for noncommercial VFR in a G-reg, but this is due to be ended by EASA sometime. Not sure if it is April 2012 or April 2014.