Here is code with debugging (i.e., the original key is retained) so that you can see the reverse sort is working. 
package com.codebits.accumulo;

import java.io.ByteArrayOutputStream;
import java.io.IOException;
import java.nio.ByteBuffer;
import java.util.Arrays;
import java.util.Iterator;
import java.util.Map;

import org.apache.accumulo.core.client.AccumuloException;
import org.apache.accumulo.core.client.AccumuloSecurityException;
import org.apache.accumulo.core.client.BatchWriter;
import org.apache.accumulo.core.client.Connector;
import org.apache.accumulo.core.client.Instance;
import org.apache.accumulo.core.client.Scanner;
import org.apache.accumulo.core.client.TableExistsException;
import org.apache.accumulo.core.client.TableNotFoundException;
import org.apache.accumulo.core.client.mock.MockInstance;
import org.apache.accumulo.core.data.Key;
import org.apache.accumulo.core.data.Mutation;
import org.apache.accumulo.core.data.Value;
import org.apache.accumulo.core.security.Authorizations;
import org.apache.hadoop.io.Text;

public class ReversingIntSortOrderWithMockInstanceDriver {

  static byte[] convert(int value) {
    byte[] key = ByteBuffer.allocate(4).putInt(value).array();
    byte[] reverse_key = ByteBuffer.allocate(4).putInt(Integer.MAX_VALUE - value).array();
    byte[] rv = new byte[8];
    for (int i = 0; i < 4; i++) {
      rv[i] = reverse_key[i];
    }
    for (int i = 0; i < 4; i++) {
      rv[i + 4] = key[i];
    }
    return rv;
  }

  public static String toHexString(byte[] ba) {
    StringBuilder str = new StringBuilder();
    for (int i = 0; i < ba.length; i++)
      str.append(String.format("%x", ba[i]));
    return str.toString();
  }

  public static void main(String[] args) throws IOException, AccumuloException, AccumuloSecurityException, TableExistsException, TableNotFoundException {
    Instance mock = new MockInstance("development");
    Connector connector = mock.getConnector("root", "password".getBytes());
    connector.tableOperations().create("TABLEA");

    BatchWriter wr = connector.createBatchWriter("TABLEA", 10000000, 10000, 5);
    for (int i = 5; i > 0; --i) {
      byte[] key = convert(i);
      Mutation m = new Mutation(new Text(key));
      m.put("cf", "cq", "value");
      wr.addMutation(m);
    }
    wr.close();

    Scanner scanner = connector.createScanner("TABLEA", new Authorizations());
    Iterator<Map.Entry<Key, Value>> iterator = scanner.iterator();
    while (iterator.hasNext()) {
      Map.Entry entry = iterator.next();
      Key key = entry.getKey();
      System.out.println(toHexString(key.getRow().getBytes()));
    }
  }
}
This code produces this output:
7ffffffa0005
7ffffffb0004
7ffffffc0003
7ffffffd0002
7ffffffe0001
I'll leave it to the reader to remove the debugging code.