I only know the theory behind lock picking but have no practical experience myself. So please take the following comment with a grain of salt.
To pick a normal padlock, first you need to put some tension on the rotating cylinder of the lock. Then enter the lock with a fine tool, delicately feel for the pins inside and push those down, each to the required level to open the lock.
Feeling which pin to push down how far is very delicate work and requires great tactile skills. Difficult enough with a lock you have right in front of you.
As far as I know the lock on a mysteel belt is positioned with the keyhole downwards. Picking a lock in that position is certainly way more difficult.
In addition, unlike a door lock, the lock is connected to the body (through the belt), meaning every tiny bit of body movement is probably confusing the tactile process of setting the pins inside. And no matter how little the wearer tries to move, stopping to breath is probably not an option. And even breathing will cause some slight movement on the belt. That is why I think lock picking - at least when attempted by the wearer - is likely to be futile.
