German passports have a field for adding your other name(s) which you use as an artist or as part of your religion (eg if you are a monk or bishop). The artist name is added if you can show that you have published work under the pseudonym or appeared somewhere) . This entry is on the paper page opposite the biometric page and doesn't appear in the machine readable part of the passport. It seems he points out that his pseudonym is shown in the passport and gets away with it sometimes