to reserve the right to read PMs in special circumstances.
I assume that you will be able to turn the function on or off.
We will never be able to install any add-on which has a feature to read private conversations (even if it can be turned off). It is very important that people in our forums can trust in that. There are already several tools (report, anti-spam, etc) to deal with private messaging abuse and also to read such reported single messages. Reading the users private conversations should be left to police or other officials.
If you want to code such a feature, please do not include it in this add-on and create a seperate add-on for it.