This only really works if rsyncing entire folders. Single files do work, however it will just appear to stand still (no progress). Also, change the path to cocoaDialog accordingly.

rsync-progress.sh

#!/bin/bash

# cocoaDialog Rsync Progress
#
# Rsync files using the cocoaDialog progress bar. Example usage:
# $ rsync -avr --progress ~/source ~/destination | ./rsync-progress.sh
#
# Copyright (C) 2014 Mark Carver
#
# Adapted from Kevin Hendricks example code
# http://mstratman.github.io/cocoadialog/examples/progressbar.sh.txt
#
# This program is free software: you can redistribute it and/or modify
# it under the terms of the GNU General Public License as published by
# the Free Software Foundation, either version 3 of the License, or
# (at your option) any later version.
#
# This program is distributed in the hope that it will be useful,
# but WITHOUT ANY WARRANTY; without even the implied warranty of
# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
# GNU General Public License for more details.
#
# You should have received a copy of the GNU General Public License
# along with this program.  If not, see <http://www.gnu.org/licenses/>.

# Create a named pipe.
rm -f /tmp/hpipe
mkfifo /tmp/hpipe

# Create a background job which takes its input from the named pipe.
~/Applications/cocoaDialog.app/Contents/MacOS/cocoaDialog progressbar --float --resize --minimize --title "Syncing Files" --text "Syncing..." --icon sync < /tmp/hpipe &

# Associate file descriptor 3 with that pipe.
exec 3<> /tmp/hpipe;

# Initialize the current percentage.
currentPercent=0;

# Read stdin, line by line.
while read line; do
  # Output rsync output as normal (for logging purposes).
  echo ${line};

  # Parse the rsync output so it can be sent to the progress bar.
  line=$(echo ${line} | awk '{
    if (index($0, "to-chk=") > 0) {
      split($0, pieces, "to-chk=")
      split(pieces[2], term, ")");
      split(term[1], division, "/");
      print (1-(division[1]/division[2]))*100
    }
    else if (index($0, "ir-chk=") > 0) {
      split($0, pieces, "ir-chk=")
      split(pieces[2], term, ")");
      split(term[1], division, "/");
      print (1-(division[1]/division[2]))*100
    }
    else {
      print "0 "$0;
    }
    fflush();
  }');
  
  # Parse the percentage number.
  percent=$(echo ${line} | sed -E 's/^([0-9\.]) [^\/]*\/(.*)/\1/');

  # Only increase the percentage if not "0". This allows the filenames to be updated.
  if [[ "${percent}" != "0" ]]; then
    currentPercent="$percent";
  fi

  # Parse the filename being synced.
  filename=$(echo ${line} | sed -E 's/^([0-9\.]) [^\/]*\/(.*)/\2/');

  # When there is no filename the percentage is parsed instead, reset it.  
  if [[ "${filename}" == "${currentPercent}" ]]; then
    filename='';
  fi

  # Output the current percentage and filename to the progress bar file descriptor 3.
  echo "${currentPercent} ${filename}" >&3;
done < /dev/stdin;

#  Turn off the progress bar file descriptor 3.
exec 3>&-;

# Wait for all background jobs to exit.
wait;

# Remove the named pipe.
rm -f /tmp/hpipe

# Exit successfully.
exit 0;