Wouldn't an escape character still be in-band signaling? It should either be something other than a string, like a NULL value, a separate field that says there's no valid plate (and to ignore the string), or have a different type altogether for tickets without valid plates.
An escape character would still be in-band, but in this context any string containing the escape character wouldn't be a valid plate (In New York any non-alphanumeric character except @ and space would work.)