The skill would be very limited to what you can do in Minecraft server side which isn't too much. I can't come up with anything myself. A big redstone lock would take up way too much space for example. And code locks are just boring as hell.
I was also thinking of maybe having a theif level that rises when you lock pick. A higher level will make the process faster.
And the thing is I hate it when people just leave the server permanently with locked chests around the map. If you try to lock-pick a chest and the owner of the chest hasn't been online in more than two weeks you will be able to do so.