unable to override /CRM/Core/Utils.php from extension

In one of our extensions we are using a new hook post_sendMail which did not make it into 4.2. We are wanting to override /CRM/Core/Utils.php for the 4.2 cycle (or until we get this into core). However, when we try to include this file in myextension/CRM/Core/Utils.php we get an error saying that there is a call to a previously defined function. If we specify this directory as the custom php directory in the browser then our code works. So it is an issue with trying to override this file. We think it is something special due to it being the config file, but we aren't sure. Please feel free to followup for more details with Pradeep Nayak. The code is from https://github.com/JMAConsulting/biz.jmaconsulting.mte.