I'm pretty sure the mod would be easy to write. I would find it difficult to live with myself if I wrote it. I got phpMyAdmin if there is ever any serious need.
Or heck, there are stupid ways to accomplish the same thing. You can get into any account easily. Just go into the 'members' table and download the SQL for that one member's row. Then change the email address to one you control and use 'lost password,' and login. Then you are in effect that member and can read all their PMs. When you're done violating their privacy just paste the saved SQL into phpMyAdmin and reset their email/pass to what it was before your ethical transgression. Yes you would mess up their unread PMs.
Or yet another way. Every wise forum operator has a test forum configured exactly like their production forum, perhaps test.example.com. Just backup your forum and up that to your test forum, then do as I said above. Nobody would ever be the wiser.