Would be nice to add a ChanSpy for MTE and have it as a script.
This feature does exist to some extent. It is the 'barge' feature in isymphony. If you acquire isymphony for your MTE installation then its possible to define users that are able to barge in on other specific users. This is the most effective way to allow 'eavesdropping' while retaining a rather granular permissions set to avoid abuse by employees.
Thirdlane/Asterisk Support available