I found this picture with google:
http://www.pierceit.co.uk/pictures/your-pictures/images/lew_papad.jpg
This comes very close to my fantasy. Getting the padlock against my will, with no access to the key. Even with good tools it may be difficult, well actually quite dangerous to get it off without the key.
so my fantasy looks like this: having sex on first date, getting bound. getting the padlock piercing, maybe even by a professional piercer that comes to the home.
then getting the rules: being a slave for some time (three months, one year..). after this time the padlock will be taken off and I would be free again. In the meantime I free most of the time, but three times a week and three weekends a month I must report to my master. failing to do so will extend the time to wear the padlock.
then there are ENDLESS possibilities what your mistress/master can do with you. even if you hated it - you would have to return and cooperate just to get the damn padlock off. or you would start to like it and beg to be kept locked more time...