サロゲートペアの文字でもちゃんとエスケープしてくれる。
import org.codehaus.jackson.map.ObjectMapper
import org.codehaus.jackson.io.CharacterEscapes
import org.codehaus.jackson.SerializableString
import org.codehaus.jackson.JsonGenerator.Feature
@Grab('org.codehaus.jackson:jackson-mapper-lgpl:1.9.9')

public class CustomCharacterEscapes extends CharacterEscapes {

    private final int[] _asciiEscapes;

    public CustomCharacterEscapes() {
        _asciiEscapes = standardAsciiEscapesForJSON();
        _asciiEscapes['/'] = CharacterEscapes.ESCAPE_STANDARD;
        _asciiEscapes['<'] = CharacterEscapes.ESCAPE_STANDARD;
        _asciiEscapes['>'] = CharacterEscapes.ESCAPE_STANDARD;
        _asciiEscapes['+'] = CharacterEscapes.ESCAPE_STANDARD;
    }
    
    @Override
    public int[] getEscapeCodesForAscii() {
        return _asciiEscapes;
    }
    
    @Override
    public SerializableString getEscapeSequence(int i) {
        return null;
    }
}

def mapper = new ObjectMapper()
mapper.configure(Feature.ESCAPE_NON_ASCII, true)
mapper.getJsonFactory().setCharacterEscapes(new CustomCharacterEscapes())

def map = ["名前":"長谷川一清","code":"<script>alert(1)</script>"]

def json = mapper.writeValueAsString(map)
assert json == '{"\\u540D\\u524D":"\\u9577\\u8C37\\u5DDD\\u4E00\\u6E05","code":"\\u003Cscript\\u003Ealert(1)\\u003C\\u002Fscript\\u003E"}'