PigJsonLoader.java

import java.io.IOException;
import java.util.Map;

import org.apache.hadoop.io.Text;
import org.apache.hadoop.mapreduce.InputFormat;
import org.apache.hadoop.mapreduce.Job;
import org.apache.hadoop.mapreduce.RecordReader;
import org.apache.hadoop.mapreduce.lib.input.LineRecordReader;
import org.apache.pig.LoadFunc;
import org.apache.pig.backend.hadoop.executionengine.mapReduceLayer.PigFileInputFormat;
import org.apache.pig.backend.hadoop.executionengine.mapReduceLayer.PigSplit;
import org.apache.pig.backend.hadoop.executionengine.mapReduceLayer.PigTextInputFormat;
import org.apache.pig.data.Tuple;
import org.apache.pig.data.TupleFactory;
import org.json.simple.JSONObject;
import org.json.simple.parser.JSONParser;
import org.json.simple.parser.ParseException;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

import com.google.common.collect.Maps;

public class PigJsonLoader extends LoadFunc {
	private static final Logger LOG = LoggerFactory.getLogger(PigJsonLoader.class);
	private static final TupleFactory tupleFactory_ = TupleFactory.getInstance();
	private final JSONParser jsonParser_ = new JSONParser();
	private LineRecordReader in = null;
	
	public PigJsonLoader() {
		
	}

	@SuppressWarnings("unchecked")
	@Override
	public InputFormat getInputFormat() throws IOException {
		return new PigTextInputFormat();
	}

	@Override
	public Tuple getNext() throws IOException {
		boolean notDone = in.nextKeyValue();
		if (!notDone) {
			return null;
		}

		String line;
		Text val = in.getCurrentValue();
		if (val == null) {
			return null;
		}

		line = val.toString();
		if (line.length() > 0) {
			Tuple t = parseStringToTuple(line);

			if (t != null) {
				return t;
			}
		}

		return null;
	}

	protected Tuple parseStringToTuple(String line) {
		try {
			Map<String, String> values = Maps.newHashMap();
			JSONObject jsonObj = (JSONObject) jsonParser_.parse(line);
			for (Object key : jsonObj.keySet()) {
				Object value = jsonObj.get(key);
				values.put(key.toString(), value != null ? value.toString()
							: null);
			}
			return tupleFactory_.newTuple(values);
		} catch (ParseException e) {
			LOG.warn("Could not json-decode string: " + line, e);
			return null;
		} catch (NumberFormatException e) {
			LOG.warn("Very big number exceeds the scale of long: " + line, e);
			return null;
		}
	}

	@SuppressWarnings("unchecked")
	@Override
	public void prepareToRead(RecordReader reader, PigSplit split)
			throws IOException {
		in = (LineRecordReader) reader;		
	}

	@Override
	public void setLocation(String location, Job job) throws IOException {
		 PigFileInputFormat.setInputPaths(job, location);		
	}
}

Yes, I have ran into this issue and I have gotten around it is by extracting each value in one step and flatten. Each value is a Tuple that you have to flatten out.
e.g.
exp = foreach all_data generate flatten($0#'request_details') as recs, (long)$0#'timestamp' as ts, $0#'user_id' as uid;
exp = filter exp by ts is not null and ts != 0;
req = foreach exp generate recs#'request_type' as req_type, ts, uid, flatten(recs#'_checkin_request_details') as loc_details, flatten(recs#'award_details') as award;