Working with relative paths in the reference editor

gistfile1.txt

import os.path
import maya.cmds as cmds
import maya.mel as mel
import maya.OpenMaya as OpenMaya
mel.eval('string $MyScenes; \
          putenv $MyScenes "C:/Users/Admin/Desktop/fairy/"')
def foo(retCode, fileObject, clientData):
  print "Callback was given %s" % fileObject.rawFullName()
  rel = "$MyScenes/"
  rel = rel + (os.path.basename(fileObject.rawFullName()))
  print "Callback changed this to %s" % rel
  fileObject.setRawFullName(rel)
  OpenMaya.MScriptUtil.setBool(retCode, True)
id = OpenMaya.MSceneMessage.addCheckFileCallback(OpenMaya.MSceneMessage.kBeforeReferenceCheck, foo)
# for deleting the callback
OpenMaya.MMessage.removeCallback(id)