We were able to edit the actual server side EUN on the box itself (backup first).  We used WinSCP to browse to /configuration/eun/en/ and then copy the respective policy to your machine.  Then use your text/html editor, and upload back to the same directory.  After the policies are all uploaded you need to enable custom ENUs.  From command line> advancedproxyconfig miscellaneous, change the value for custom EUN pages. We were able to create some great looking messages with helpful troubleshooting information.  This might be a little tough to follow, there is a support doc out there, ask your rep or support.