Saintprudence That sounds like the setup to half a dozen permanent chastity stories. “Oops, I locked it on, now where are the keys?”
If it has an integrated lock, chances are the keys are necessary to close it. If it has a padlock, best to leave the lock with the keyholder as well. I mean, if they are keeping the keys away from you, it is best for them to demand the lock remains with them as well, as a lock in your possession contains all the information necessary to make a spare key. And the now-no-longer-involuntary wearer can buy a new padlock of the same size to get a well-fitting belt with keys for personal use, whether for their own lifestyle choices (e.g. using time-lock safes or wearing it only while visiting bars or something) or for a future partner to hold the keys. The biggest issue would be an enclosed padlock that is permanently held to the belt via rivets or welds, but the MySteel padlock looks quite straightforward to take off.