I'd say this is on the mid end... to run something every few minutes requires something called a cron job. It requires having access to the server to setup.
After that, the actual script would be pretty easy as long as you have the access information for the database and the database allows foreign connections (a lot of databases don't allow connections from outside the server they reside on). If your DB doesn't allow foreign connections, there are other methods, but more convoluted.
Why not edit the code that sends the PM? I can understand concerns about upgrades, but as long as you keep track of what you're doing, it may the best way if you don't have a great deal of access to the server.