Ok I am just going to use this picture as reference, but there is no way that a Japanese plate will ever read vegetable truck.



becuase The top line is the name of the issuing office and a vehicle class number, 大阪 this is written in Kanji and this plate is from Honshu, Osaka. The bottom line has the hiragana character and a four-digit serial

So no, there are no customized plates in japan, and it is actually quite hard to get a real Japanese plate out of the country unless you steel it off another car because japan is very strict with their plates and it must be destroyed at the police station before you are issued a new plate. And cars that are junked with the plates on them have to be destroyed and turned in or else they face a very big fine or jail time.