Interesting stuff. Im not opposed to someone using system tables if they know what they are doing. The downside I see is not that something might break (hey, you changed it!) but that it may get undone by a service pack (will you remember to check and reapply your change) and that it you're doing something not expected/easily visible for the next dba to come along.
I don't have a jr dba - idea kinda gives me chills! I think when you get to the point that you don't trust them, time for them to go. Note that I make the distinction between trust and worrying that they may make a mistake. Mistakes will happen, especially if you're learning. I think the thing to do is teach them to never do things without a recovery plan. Never never never. Maybe invest in LogExplorer too!
Andy