You can hide a simple inverted J wire onto the back.
You can make an attachment that slides over the pinstem with a bale
attached to the tube. Have a spring mechanism so that the attachment
isn’t floppy. A simple leaf spring that catches near top and bottom
of piece works well.
You can make the pinstem and bale as each removable attachments,
typically they’d screw on. If there is enough clearance you can leave
the pinstem permanent.
You can use an external pin setup (like a fleur de lie (that was
popular for pendant watches for awhile eons ago) that hangs the
pendant below the ornamental pin finding. A simple swiggly hook works
to catch the bale.
Any of these can be on the simple and cheap or be as relentlessly
crafted as you stand.